My guess is that he made a quick appearance at the party simply so he could get his last word in to these folks. I also suspect that the third person was outside and called or texted them as soon as cops started to respond, allowing them to make a clean escape.
The cops just happened to be on a training exercise nearby and responded quickly... had that not been the case, the shooters would have had several more minutes to do their thing before having to bug out.
Probably a microaggession by someone at the party. Like a disrespectful glance.
The logical explanation was that this muslim scum was on edge because he knew that it was only a brief matter of time before he was going to go all jihad on the group in that room.
I'd say that it was fake anger, a ploy to allow his victims to relax before he returned PREPARED with wife. Drop 6mo baby with grandparents, dress in combat gear, grab weapons and bombs then return. How much time for being 'angry?'
Besides, this was an office holiday party so there was probably whispered mention of Chanukah, Christmas and Kwanza. Probably not a single mention of the birthday of a 7th century leader (Christian Calendar). (Remember Islamic holidays roll to the lunar calendar, not the same on our calendar which adds to the nuttiness!) Question: Are fir trees enough to set off jihadi anger?
The Christmas party probably had ham, pigs in a blanket, bacon, pulled pork, etc. ;)
That would probably be enough for an Islamic ticking time bomb - which ANY ONE of them can be.