Without seeing the rifle I can't say for sure, so to be safe, my suggestion is that you have the guns checked by Marlin to be sure there is nothing significantly wrong.

A theory:

I suspect recoil acting against the inertia of the lever can be enough {particularly on a slick-actioned, or older, well-worn actioned rifle} to cause lever movement. Actually, such movement would not be lever "movement", but rather, the lever staying put while the rifle recoils away from it, with the weight of the lever being enough to cause partial unlocking of the action, all of this taking place after the bullet has left the barrel?

Thus, recoil speed, not pressure per se would be the "culprit".

I believe this is known to occur a bit with Win 94's, etc, also. I have seen bolt handle movement on Lee-Enfields and it was explained to me that this is the cause of that, so maybe the inertia thing is the cause of both?

How hard to you have to hold the lever against the stock wrist? How did you come to notice it?

Try this:

UNLOAD the rifle.

Cock it.

Aim in safe direction.

With just your finger, trip the trigger. Do NOT touch the lever.

Does the lever drop just a bit?