Last week we had 5 of 6 teams delete themselves without warning or explanation, all within a minute of each other. They are currently "soft deleted" with an option to recover in the admin portal. The Teams had different owners and no one can explain what happened. Searching the audit logs shows no record of a Team, 365 Group, or Team Site being deleted. We're still in the piloting phase of Teams and if we're going to adopt it more widely we need to find out who / what deleted the Teams and how to keep it from happening again.
Use the audit log in the Security and Compliance centre, it will tell you who deleted the Teams.
I've had users who didn't realise they were owners delete them by accident when they mean to leave the team. It's a bit hard to see why, the messaging is quite clear. I think it would help if Leave Team was at he bottom of the options rather Delete the Team.