pH is HUGE in cheesemaking. pH variances of even .1 can make a large difference (I've posted before here about why this is). For example, drain your whey at 6.2, you get a smooth, cuttable cheese, drain it at 5.8, and your cheese will not hold together very well and be more crumbly.
Mozz is no exception and is especially picky. Your milk set, so it doesn't seem like it was UP. Seems like there was too much acid and the curds fell apart when you heated them. Every time that's happened to me with mozz in the past was when I either went to sleep and the curds overacidified with cultured mozz, or I added too much acid.
Milk has an isoelectric point of pH ~4.6, and the closer you get to it, the worse the stretch is. Technically, a cheese will start to be meltable at a pH of 5.8 and stretchable a little below that.
Salt without iodine is best.