Do Bees fight?

I have a dead tree stump which for the past couple of years has had a bumble bee nest. This year some small bumble bees began a nest and around the other side a larger species of bumble bee began another. Wonderful I thought, lovely to see them whizzing in and out.
However over the last few days I've found several dead small bees at the base of the stump. Does anyone know why this may be? Do bees fight between colonies? Or might it be that the little ones went into the wrong entrance and were attacked?
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So many possibilities. Is there a lime tree flowering nearby? Lime tree nectar can kill bees. Or their nest may have been parasitised by another insect, such as the wax moth or even one of these
http://www.nhm.ac.uk/nature-online/species-of-the-day/biodiversity/loss-of-habitat/bombylius-major/
Do have have any photos of the dead bees so that someone here can identify them?
The worker bees don't actually live very long, and if they die or are unwell in the nest the other bees will remove them and deposit them outside to remove danger of infection etc spreading.
Lots of info here http://bumblebeeconservation.org/about-bees/faqs/finding-dead-bees/
Thanks for info pansyface. There are no lime trees around our garden at all. I just wondered if the bigger bumbles were killing the little ones. My computer is playing up so can't upload photos, but they are definitely two different variety of bee, or is it bees!
Dove's idea of sick bees being removed from nest is a good suggestion, though there seems to be too many unless the hive is infected.
I'm not very up on various bees, however I witnessed a couple of times last summer a European wasp grab and kill a honey bee.