Thanks for the sharing your experience on Chair @Eli Spitulnik. The technical climbing is often the easiest part of a Cascade alpine adventure, especially in the winter; and finding the descent gully/rap stations off Chair is not trivial.
I have seen many slides on that slope your partner triggered - the loading is often way different on that climbers left side of the upper Chair Peak basin compared to the slope you ascend on the right to start the route. As soon as one enters the Source Lake basin the avalanche terrain exposure quickly ramps from challenging to complex and increases to straight up complex in the Chair basin - basically most of the approach/descent is one big terrain trap, so the forecasted avy danger for given day doesn't mean that much in that zone. Great TR! Sharing your experience is very educational.