Providing all was well before the oil change as stated the only way I could comprehend this happening is
A. the pushrod just happened to snap or bend at that particular time.
B. the lifter, if hydraulic may have collapsed enough before oil pressure pumped it back up.

I do find it a bit strange and even more so that the lifter jumped out of the bore unless one was winging it which is unlikely.

Finding the push rod may help determine the cause.