Ah, how familiar that sounds! Went through it (twice!) with my now 28 month old daughter. There was absolutely no triggger . . . she was just suddenly terrified and crying hysterically!
I put on my bathing suit and climbed in the bath with her. She was still nervous, but less hysterical. Do that a few days in a row, then try a solo bath again. I also moved her to every-other-day bath, but that was during cooler months when she wasn't outside getting dirty/sweaty.
She still has issues with her hair, but only because she hates to have water dripping down her face and into her eyes. So I always lay her on her back (which she is now fine with) to wet her hair, then to rinse out the shampoo, using my cupped hand or a small bowl to rinse the unsubmerged parts of her head. This way the water doesn't run down her face/eyes (except when I'm careless and splash!).
The good news is my daughter got over it -- in less than one week the first time, but almost two weeks the second time. She's likes to play in the bath, and absolutely loves the pool!