The rocks these fossils are associated with come from have some volcanic influence. The most famous example is the Messel Pit, which isn’t where these fossils are from, but it’s interpreted that events similar to the Lake Nyos eruption might be to blame for fossils. At the Messel Pit there’s a few turtle fossils which also died during sex, so it’s really just an unfortunate coincidence that they were having sex when it happened.