What causes valves to stick?

[SavageC20]

Originally Posted by HO455

Are the pictures from the first incident or the second time? I would look very hard at the oiling system. Inspect the oil pump and its relief valve. Pull the filter and cut it apart to see if it is plugged. Check the filter bypass. Pull the galley plugs and rod the passages if necessary.
Low oil volume will cause the valve train to overheat. The oil that flows from the push rods down over the rocker arms valve springs and the down to the valve guide is what cools those components. If the volume of oil is not enough then heat builds up. The heat causes the guide clearances get tighter which in turn makes more heat. Worse case is the valve siezes and it is hit by the piston.
The gauling on the push rods in the photo look to me to be oil related.

The pictures of the pushrod and the cam lobe are from the first time. They rodded the oil passages when it went in for the second time. engine has melling hi-volume oil pump. they were unable to find any issue with the oiling system. filter was cut open and was clean.