IMO, the two "most likely" scenarios for the pushrod coming out are.....

-if it has adjustable rockers, the adjuster backed off until the pushrod fell out.

-the pushrods are not quite up to the job at hand, and one of them crumpled under the stress

68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads