I feel like people aren't giving the MC enough leeway. Remember, it's been... weeks, at most, since his ex-wife framed him as being abusive while divorcing him. And as this chapter shows us, he's always scared other people with his size and face. But let us reiterate: the woman he married and presumably had a years-long relationship with blindsided, betrayed, and then slandered him. That is the kind of thing that takes a person years to recover from, if not outright turning them into a misogynistic incel.
And now, his memories of his childhood friend supposedly betraying him have come rushing right back. Meaning that hurt is fresh and freshly remembered, compounding the above and his inability to figure out for himself how the FMC feels about him. The guy is positively plagued by uncertainty to the point he can't even trust himself or his own judgment. Especially since the last time he attempted to pursue or confront a discussion/argument... gestures to the aforementioned wife incident. So it only makes sense that his current default when it comes to conflict is to disengage, because trying to confront it all but ruined his life.
As for a child managing to cause all that destruction... well first of all, shelves might be heavy but it's certainly not impossible for a child to push them over. Secondly, he himself was a kid, and she apparently never cleared up the situation. So that whole thing was essentially allowed to fester. Sure, approaching it from an adult's logical perspective would point out some holes, but the stuff we carry with us from childhood we tend to still look at from that point of view. Especially since he pushed down those memories for years now, so he never pursued the subject or thought about it at length.