One time I was switching between phones or restoring from Titanium Backup, and WhatsApp removed me from all my groups (maybe when I logged on from a new phone, I got a new key, and when I restored using TB, the new key didn't match my key the members knew)...

If someone from one of the groups confront you/GP about leaving, you can always say "I was restoring from backup and it looked like something went wrong!"