This is an interesting theory, apparently by British zoologist Andrew Cunningham (link from ORF article).
Couldn’t find an English version at a pinch, so just a little TL;DR:
Bats have coroaviruses that usually lie dormant, pretty much the same way as lots of humans have dormant herpes viruses which will “wake up” under stress.
The viruses have adapted to the bats, obviously. When bats fly their metabolism ramps up, leading to their body temperature rising to levels that would be a light fever in other mammals.
Man intrudes into bats’ habitat, bats are stressed, viruses “wake up”, bats start spreading viruses.
[IMO, there probably is another step inbetween, the virus aquiring the ability to feel at home in humans.]
Humans catch viruses from bats, viruses can survive elevated body temperatures, usual reaction from humans’ immune system doesn’t work, at least at first.
Thoughts, anyone?