do you keep bees yourself John? i'm guessing not. It's really good of you to be concerned and try and report it to ind out why. Most people wouldn't bother and just brush them away. I'm new to bee keeping so can't help with your answer but I'm sure others on here will be able to advise you. You wouldn't believe the amount of reasons this could be because of.
Is there anyone who keeps bees nearby that you are aware of? and it would be useful to know the answer to the question Hivemaker asked, how many?
Not everyone knows this stuff.....I didn't until the start of this month. This guy seems like a concerned member of the public, who has found lots of dead bees, and rather than ignore it, he's tried to get help in case there is something going on in the area. I have never seen a lot of dead bees in one place. I would have thought the same, maybe someone had poisoned a hive. It'a good that people who aren't interested in keeping bees are at least looking out for them, even if they make mistakes.
BTW, while snooping on website looking at what bumble nests look like, I saw one with a queen on it......YIKES!!!! If I saw that flying towards me I'd be back to square one! ( even though i know bumbles very rarely sting and are very gentle )
Heres a question for you all, just for my own sanity.....do bumble queens fly to mate like honeys?