It’s a well known bug. It occurs when you destroy the shards while the Old Sentinel is in mid-air.
The working (player) theory is that, as soon as the shards are destroyed, the script tells the Old Sentinel to jump off of the platform; since he’s in the air and incapable of jumping, it breaks his script, making him stand still and tracking on whichever player he happens to have aggro on. Because his script is broken, it never removes his invincibility so you can’t damage him to get his script back on track (like you can with the various broken script Thrall that will sometimes appear, just standing there doing nothing, or against either of the Guardian mini-bosses when you do fun things while they’re jumping so that they do the same thing as the Old Sentinel except they aren’t invulnerable).
It’s been a while since they’ve commented on this (afaik, it’s around since release) and they haven’t done any of the “easy” fixes (having the shards appear only after the Old Sentinel is on the platform; having the shards be shielded until the Old Sentinel is on the platform). Kinda curious if they’re trying to fix the script itself or simply have it on the back burner (since, once you know about it, it’s pretty easy to avoid since you just don’t destroy the shards until he’s on the platform).