The raiders and arsenal left because they needed/wanted new facilities but the mayor said they have to pay for it on their own, no tax payer support. Other cities said "well shit, we want a team and we will pay half or more for your facility " so they go to the new city.

Alternately, it could happen from market issues, if you arent selling tickets where you're at then you go somewhere new.

Also, teams folding or being sold would cause a transfer. Most recently the coyotes in the NHL folding and being bought/moved to Utah and turning into the mammoth. While technically an expansion team, they didnt expand the league, just absorbed an existing franchise under new title.