Does anyone know if this is normal behaviour please?

This afternoon I had the pleasure of seeing a Kingfisher on my local river. It has had 4 young ones fledge this afternoon.

What I wasn't expecting at all was this:

Two of the fledglings began to fight, and one got the other one on the floor, managed to flip it over on its back, and continued to fight! The tussle ended and the one on its back managed to get away. However it happened again a few minutes later, but this time the aggressive one got the other on its back again, they continued to fight, with the one on its back making awful noises, they fell into the River and continued to fight, which ended up in the aggressive one flying away leaving it's sibling dead on the river surface!!!

There was a pair of Grey Wagtails nearby who kept going near wondering what was going off with all the noise!