Assume a bloated ontology, on which there are events, chairs, holes, waves, etc. In defending such a bloated ontology, we should sensibly say that these beings are grounded in what the fundamental beings are and how they are, and so the bloat does not infect fundamental reality.
So far so good. But what if we add presentism into the mix? Imagine two worlds, A and B, that are exactly alike at time t, but in one of them a race is beginning at t and in the other it is ending at t. We may here suppose massive mental dysfunction in B, so that everybody thinks the race is beginning, whereas in fact it has just ended.
Then in A at t, there is an event E that doesn't exist in B at t: the beginning of the race. Suppose t is the present moment and A is actual. Then E is not grounded in what the fundamental beings are and how they are, given presentism, since the fundamental beings and how they are are the same between A and B. In our bloated ontology, there really are events, and E really does exist, though. And so we have a violation of the principle that all beings are grounded in the fundamental beings and how they are.
Some presentists will get out of this by saying that A and B differ in the past, and facts about the past are grounded in what God remembers. Note first that this requires a denial of divine simplicity. For, given divine simplicity, there cannot be two possible worlds that differ only in how God is, since that would imply an intrinsic accidental property in God. Second, we have the standard criticism that this gets things the wrong way around: God remembers the past because it was so, rather than its having been so because God remembers it thus.
The above is a slight tweak on the usual grounding objection to presentism. But I do think that while one might balk at the claim that all truth is grounded in what fundamentally exists and how it is, it is plausible that all being is so grounded.
The best way out for the presentist is to deny the bloated ontology, of course.