It was definitely due to the back brake. This is because the front wheel is stopping abruptly and causing friction and therefore stopping momentum, whilst your back wheel is still going full force.
This means your front wheel will be pushing down and gripping to the floor whilst your back wheel will still be moving therefore meaning the back of the bike will fall forward.
Include that with your weight, it results in a very painful fall!
Anyway, i hope you are okay mate. Sounds like a nasty accident!
Hope this helps