Good movie, but question about the river scene (spoilers)

So...Joaquin's character gets excited that the glow is fading, runs and grabs a jug of the pre-diluted mixture, dumps it in the water and ends up killing the other two guys.
I was a bit distracted during this part, is that about right insofar as him just losing his mind for a moment there?
Okay, but then...why did everyone go jumping into the river? Joaquin was above it, standing on some ledge or something, right? He dumps the stuff...and suddenly everyone's in the water, screaming for dear life. But...why did they jump in the water? Trying to get to Joaquin?

The movie was great, but I don't really get that part. Everyone's great, its happy gold-picking time and then BOOM, Joaquin goes temporarily insane (apparently forgetting that there's plenty of mixture, a lot of gold and more than enough time to get it) and everyone's screaming bloody murder as they burn to death.
That turn in the movie would have bummed me out anyway, but to have it make little sense just makes it that much more of a disappointing turn for a movie that was otherwise really immersive.