Working as intended. The tanks had aggro on the bosses, tanks went up, bosses evaded. Do more dps on Brontes so that you have the full time to go get the crystals before the bosses become active. Or, if this happens again, forget the crystals and just continue as normal. You'll just have to actually do your tank rotation to hold aggro sans crystal.
As I said, most of that group did not clear it and wanted to see how it is in HM. It is not the usual progression group. I have done that fight so many times successfully and his is the first time I notice this feature. Funny enough we had bad attempts where we had aggro as tanks, went up but they did not reset. Could be something new similar to Calphayus if you don't taunt him in the final phase he resets.