I found that weeks ago when i entered the tunnel that bring you to the boss and he was on the other side of the door waitting. Had to fight a Blood Bishop and the Butcher together. Was a pain, but was able to kite them with Bone Prison spam for the win.
The answer is killing boss first.The Butcher is a true tank.
Had this happen to me last week.Went into Sarat's Lair, didn't explore the boss room as I went around. Got into the boss room, saw the spider boss descend AND heard the famous FRESH MEAT.Fun times were had.
Could have him act like the Bazelgeuse and just show up randomly on any boss fight. Maybe not even always at the start.
Besides next to a damage or immune shrine in the boss room is the best place for him to spawn.
Ive encounted the butcher twice and one of them he was mandatory to actually clear the dungeon. I was in my 40's as I was leveling when I encountered him twice. lets just say I didnt kill him either time and I gave up the second time because he was too much to handle at 40 something and I wasnt strong enough.