Sudden arrival of Shenshock and Allies Mid Battle...

on my latest foray into the Temple I was fighting Barkinar mid battle when Shenshock and his two Bugbear guards seemingly teleported into the fight. Shenshock showed up in the lounge while his 2 buddies decided to stroll in on the west door from the troll room.

I'm guessing the timer just happened to trigger during the fight but it was strange. Realistic to have them show up as reinforcements though.

So my question to any who wish to reply, has this ever happened to you?