There are all kinds of ways to kill a valve, and most of them -- like cracking, warping, flaking and pitting -- have to do with heat. While burnt exhaust valves haven't actually caught fire and "burned," they're said to have burned when they've lost their ability to seal the chamber through some kind of thermal damage. But, in something as delicate and complex as an engine, this kind of localized overheating can happen for any number of reasons.
Valves, particularly exhaust valves, get very hot. Exhaust valves pick up most of their heat while they're open, when thousand-degree exhaust gases are compressing and flowing around them. They shed about 75 percent of this heat into the cylinder head through the valve seat while the valve is closed. Excess heat in the valve will damage the metal's crystalline matrix, causing cracks that range in scale from microscopic to visual. At the smallest level you have pitting and erosion, followed by chunks of metal flaking off and large visual cracks in the valves. Such damage often causes a cascade effect of failure, since the valve or seat loses its smoothness and surface area and thus its ability to transfer heat.
Exhaust valves face wear just like everything else in the engine. While exhaust valves are very tough by design, they're also subject to constant and extreme heating-cooling cycles and pounding on the valve seats. The heating and cooling cycles ultimately anneal the metal, softening it and allowing bits of it to break free. Combined with corrosive chemicals and heat in the exhaust, this cycle eventually causes the sort of pitting you'll find on any valve with enough miles on it. A normal valve grind operation should smooth the valve and seat, restoring the seal.
Your engine could burn quite a bit more fuel than it actually does. The average engine runs at a ratio of about 14 parts air to 1 part fuel, but this is a bit heavier on fuel than is usually necessary. Engineers design the engine to use slightly more fuel for a few reasons. The first is that more fuel in the cylinder means a denser fuel charge, which means a more stable burn and less chance of a misfire. The second reason has to do with cooling, since burning less fuel produces more heat. An engine tuned to run very lean can potentially get a bit better fuel economy but will probably melt into a puddle of aluminum, offsetting the money you've saved on gas. Lean conditions can happen because of a malfunctioning injector, low fuel pressure, bad carburetor tuning or too much turbo or supercharger boost.
Here's a gearhead legend with some basis in reality. According to old men with beards and cigars, running a car with open headers -- sans any sort of exhaust system -- will allow air to back up into the engine, cause it to run lean and burn the exhaust valves. This is true and it isn't. Air backs up into the engine because of pressure reversion, which is what happens when pressure waves reach the end of a tube and bounce back toward the engine. Headers using individual tubes and a collector where the tubes join are engineered to avoid pulling in gases from adjacent cylinders, so this scenario is extremely unlikely with open headers. "Log-type" cast iron manifolds, on the other hand, are prone to sucking gases back in from outside the manifold at certain rpm, because they merge those pressure waves right next to the cylinder head. So you won't burn a valve with open headers, but you might with a log-type manifold or, worse, no manifold at all.
Performance enthusiasts often specify a multi-angle -- three- or five-angle -- valve cut when rebuilding an engine. A valve guide with a single, flat angle causes air flowing through to turn two fairly sharp corners before going into the cylinder. The resultant reduction in flow typically manifests at low rpm and low valve lifts, when air has to punch through those small high- and low-pressure pockets to enter the cylinder. A three-angle valve job consists of re-cutting the edge of the valve seat to present a more rounded channel, which boosts power primarily at low rpm. But a multi-angle valve job reduces the contact area between the valve and valve seat, reducing the valve's heat transfer ability. A multi-angle valve job won't necessarily cause valve burning in and of itself, but it drastically reduces your margin of error where heat is concerned.
Here's one most people don't think about. Valve recession isn't a failure in the valve as much as a failure in the valve seat. Over time, your engine's valve seats will either wear away or get slowly pounded into the cylinder head. When that happens, the valve ends up having to move closer to the cylinder head to close fully. Which is fine, except that the top of the valve, the part in contact with your rocker arm or cam follower, ends up moving closer to the rocker arm or cam follower in the closed position. Sooner or later, the valve will permanently wedge against the rocker arm or follower, causing the valve to hang open instead of closing all the way and transferring its heat to the cylinder head. This is part of the reason that periodic valve lash adjustments are so important on engines with solid lifters, which are somewhat more prone to this kind of failure than those with hydraulic lifters or lash adjusters.
