Well, the pH may be a sort of co-factor in some cases. But usually not the root cause. Sort of like a red herring, in that we might say, oh the pH is too low, that's why the knit is poor. But what we really mean is that the temp is too low, causing slow pH drop, and with the temp being too low, the knit is poor.
What happens at a physical level during fusing is that all curds have a thin (or in poor makes, thick) membrane that has dehydrated. There's a sort of gradient. When there's contact, the curds begin to bond together through calcium phosphate bonds. These bonds form better when there's at least some moisture left (small shell), and when there's not excess fat, and when the temp is conducive to creating the bonds (75F+). The outer edges press together and fuse, hopefully incorporating minimal gradients through the cheese mass.
If you look at a solid slice of cheddar in a microscope, you can see all the curd fusion points. They look a bit like cells. Clear fusion lines of low moisture "skin" all stuck together through calcium phosphate bonds.