Bumble bee mating madness ??!!!

Last week I thought I had snapped bonking bumble bees! Some said no, there is no way, this too rare a sight to ever catch, and some said well done to catch such a rare event.
http://forums.dpreview.com/forums/readflat.asp?forum=1042&message=41680227

So I posted pics off to the site suggested by Matt, and sure enough they confirmed it was what I thought it was!

However, does lightening strike twice? - a week later I was amazed to see the same sight at exactly the same spot!
I am no bee expert - bees playing piggy back rides or procreating?

A jet of clear liquid did shoot out of the rear end - if you look closely you can see a droplet.
Also the queen has something on her rear leg?

I would be very interested in any thoughts as to what is going on!