What causes valves to stick?

[SavageC20]

I rebuilt my 454 in my 72 last summer. Ran the engine for about 600 miles and then started getting a lot of valve train noise. Pulled valve cover and the tips of several pushrods were worn down/galled. Replaced pushrods that were worn and fired it back up only to hear a horrible noise in the top end after about 20 minutes of running. Pulled valve cover again and discovered stuck intake valve . Pulled intake and 7 or 8 lobes of the cam were pitted, but the bottoms of lifters were fine. Pulled engine out and paid machine shop go through it again and I also had them break the engine in this time. They said the valve hit the piston and put a nice ding in it , but they were able to reuse it. I go to pick up the truck and two miles down the road I hear the same noise and now a different valve is stuck. Keep in mind they went through both heads and installed new seats, valves, and springs the first time I rebuilt the engine. I'm not sure if it's the same head this time.

Engine is 454 bored .40 over, fresh 049 heads, .478/.501 cam, flat top pistons

What can cause a valve to stick?