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So with what little info we have to go on, I was thinking about how Dynamax might work in a battle and the balance the creators intended. Now the three turn limit of Dynamax is what immediately stands out, but I think the move Max Guard is also intended to restrain the power of Dynamax.
From what we've seen Dynamaxing turns all your moves into new "Max Moves" which differ by type except regarding status moves, which all turn into what appears to be a sort of super protect. I'm guessing that, given the strict turn limit on Dynamaxing, Max Moves will be able to deal piercing damage through Protect in a way similar to Z-moves. This is to help prevent your opponent from robbing you of two of your three turns by Protect stalling you out. This is where Max Guard enters the picture. I'm guessing its most straight forward function will allow you to completely block both Max and Z-Moves. But I also think the fact that it replaces any and all status moves is meant to curtail the power of Dynamax.
Imagine a Zygarde coming in after a screen lead and instantly Dynamaxing for a huge HP and/or stat boost, but instead of using these turns to attack, it just uses the free turns of additional bulk to set up Dragon Dances. This sort of behavior is blocked with the introduction of Max Guard. Basically Gamefreak's way of saying that Dynamaxing is for attacking, or countering your opponent's own Dynamax; not for guaranteeing a setup sweep of your opponent's team.
TL:DR Max Guard replaces all status moves to prevent players from using Dynamax to set up.
