I think most LO's go thru this somewhere between age 1 and 2.....and we'll neber know why it happened, but it happened for us tOO! There are a few things you could try....
Water in a big rubbermaid bin instead of the tub. Standing (not sitting) in the tub might help them feel more in control. Getting in there with your child...(this is what worked for us). Bathing in kitchen sink (if there is room!). Blowing bubbles, tub crayons, special cool things that are *only* used at bath time. My friend got one of those automatic bubble blower thingees, and the whole tub area was filled with bubbles, and her ds wanted to get in just to touch them. It will pass....we just tried not to turn it into a battle...we felt that doing that would just make it last longer, kwim? So we were patient and calm...and did what we had to do for a couple of weeks, and then it was over as suddenly as it began.
(((hugs))) It is hard when they're scared.....but I promise this will pass!