I have six bees on the lawn, bumble-type bees, big fat hairy stripey ones,
and all quite dead.

Considering that I have never ever seen one dead bee on the grass before,
six at a time seems a bit coincidental.

Are there any bee-people (Mary?) who would know why this has happened all of
a sudden?

It has been wet, but not that wet. My feeble brane says that it is just
possible that bees are flying along, in defiance of all the laws of
aerodynamics, and get caught in a downpour, get waterlogged, and just fall
out of the sky. But that sounds a bit far-fetched.

The bees show no signs of having been mauled by an animal or suchlike.