Large sps colony dying from the bottom?

[Animal-Chin]

I have a huge birdsnest colony smack dab in the middle of my tank. Its Gotta be 3 years old and has grown from a tiny frag to the monster it is today. So last week I notice some white at the base but didn't think much of it. Now, entire branches are dying off over night. Its happening fast! I've clipped off the dead stuff to try and prevent spreading but anyone know what could cause this weird shift?

Only factor in my tank that's changed is I've pulled 5 fish to reduce bioload and have water changed my tank aggressively for maybe 2 months to bring my nitrates from 10 down to 5. I was changing 20% weekly.

Could it be a pest attacking?

Parameters

Phos - 0
Nitrate - 5
Cal - 450
Alk 10.2
Temp 80

Everything else in the tank, my acros, monti, lps, zoas, everything is totally fine, its just this one big birdsnest...