I've wasted more than half an hour searching Help for this - I changed one transition to a spiral or something similar, and now, without me asking for it, Vegas is using this same transition every time I overlap two events.
Check out pages 136 and 137 of the Vegas Pro manual which carry details about turning on or off automatic crossfades and, probably more specific to your situation, page 137 which covers 'Converting a cross fade to a transition'. Though I must stress that I've never used this procedure, you might be able to use this same process to revert to a default crossfade if this process was inadvertently initiated to create the problem.
Hope it works for you.
Edit: I've played around with the above and it doesn't after all change the default from a crossfade to a transition as far as I can see. Sorry.
Peter, have you tried selecting the events with the transitions and then drop the crossfade onto one event? This should change the transitions for all selected events.
Thanks for the suggestions. Yesterday, after 24 hours of every overlap (in this particular project) becoming a "Spiral", it spontaneously reverted to being a crossfade. I checked the events either side of the overlap, and they were the same as the previous auto-spiral, so I don't know why, but I'm relieved I don't have to manually change every transition.
This project is a multicam, so each event is in fact a "pile" of 6 Takes - don't know if that makes a difference, but thankyou Vegas for behaving well again!