I wonder about the choice of venue too. How...odd.
Appalling breach of decorum, period, but I don't understand the musicians applauding either. I hope it was a case of "Screw it--if you can't beat 'em, join 'em." I have performed many times with a symphony and in chamber settings and, of course, attended even more performances as an audience member, and cannot imagine ruining a concert in this way or anybody validating behavior such as this by applauding.