I've read quite a few stories over the years and have heard multiple answers, as such I might be misremembering/mentally combining some articles. Overall, the general consensus is a combination of issues. Just earlier this week, there was a report about mite infestations in bees contributing to it. Pesticides seems like a likely cause as well. I remember as a child there were many in my area going willy-nilly with all sorts of chemicals to rid the neighborhood of bugs. Several used to fog their entire yards to get rid of the mosquitos. There was another article, this one I remember much less, that was pointing to something technological was disrupting the bees navigation capabilities which results in them flying around like drunken pigeons with magnets taped to their head. It used research on how colonies that were in the process of collapsing had bees in it that were erratically "dancing" to guide other bees to pollinators.