I'm pretty sure this is happening because your tanks are pushing the actual boss too far, it popped up in another thread pretty recently. Unfortunately, with the small health pool in 8-man, its easy for the tanks to push too hard, forcing the boss into a hard enrage instead of his regular tantrum. The mechanic was put in place to make sure groups don't just ignore the tentacles and focus fire the boss, but it was tuned around 72-gear. Make sure tanks watch the boss percentage, and if needed, restrict their rotations to use only the abilities needed to maintain defensive buffs.
Edit: Here's the link to the previous post of other groups encountering your same problem.