Why do valve seats fall out?

My truck is acting funny lately, and it sounds a lot like it did when I had a valve seat fall out 2 years ago. (Rough idle, ticking because the valve can't close all the way)

What can cause a valve seat to fall out? Any ideas? Could it happen depending on how the engine is running (lean, rich, hot, etc) or do I just have bad head?

I got a long block 4 years ago, so I have 40k miles on the block and head, and about 220k on the rest of the engine (fuel injection, emissions, stuff like that). I do not think I should have to deal with *2* valve seats falling out in 40k miles, am I wrong?
 
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