Occasionally we see this problem on a push mower and over time the engine bolts can loosen up to the point that the engine starts to move. The bolts can actually fall out as well. There are normally 4 bolts that hold the engine secure to the mower deck.
Loose bolts can also cause the engine to torque in the deck and that action can crack the engine sump cover. The sump cover is the lower part of the crankcase and their bolt holes that hold the engine to the deck are located in it. You can also over tighten these engine bolts and crack the sump cover, so be care of how much torque you apply to it.
The problem with finding loose engine bolts is that the deck is covered in grass and you cannot see them. Scrape the deck and find the engine bolts and check them for the proper torque. The lawn mower manufacturer should be able to provide the torque specs for them.
A loose lawn mower engine is a safety issue and needs to be corrected to keep your lawn mower working in a safe manner.