I baked my mother's chocolate cake recipe (which she only frosted in the pan and served) and tried to layer it. I shaved the top off to make it straight before adding a marshmallow center and the second layer of the cake. After I cut the cakes first piece, the rest of it slid off the sides. Can it be because the cake is too moist to hold up the weight, or is it the marshmallow filling?

How long did you let it cool before leveling and layering it? My guess would be that you need to let it cool longer. Good luck!! 

Yes, there are some cakes that can be too moist to layer. But since we dont know about your cake recipe, we cant say for sure.

If you mean that the top layer of cake slid off the bottom layer of cake- then I would say it's your filling, as in you may have had too much or it wasn't thick enough.  I'm not sure what moistness would have to do with a cake layer sliding. 

I agree it's not the cake it's filling.