That is a shame, can't say I've noticed anything similar. According to bumblebee.org parasites are the major predators of bees.
Worth having a look to see if you can find more info.

The queens and drones of Bombus species are in flight now and it is a pretty vicious affair for the drones. Quite possibly they are mated drones that have been mutilated in the struggle. They have evolved to die almost immediately after mating anyway.

The young queens also kill the older ones if they can.

Gosh I've never seen this before - thanks Buddlejagar!

Nature is cruel. As soon as the killing starts on the
David Attenborough programmes I switch off.

Lime trees flowered about a month ago. The nectar is important to bees, especially honey bees, but it has a narcotic effect, and many bumble bees die as a result of feeding on it. The time scale is about right. I expect what you saw was the after effects, and other insects (especially wasps) have benefitted from the windfall, clearing up the dead bees, obviously taking their juiciest bits (!) leaving the crunchy bits for you to ponder over

So, possibly a more complex reason than I first thought. Thank you Buddlejagar and Worthy1 for your inputs.

It's a rough tough world out there. We've got it pretty easy. Not to be a drone, eh, second time round!

Thats interesting about the Lime Trees. Do the people
researching the loss of Bees know about this ?