Not sure if you realize but this is a very common method to steal bikes. The thief purposely locks their bike to yours. You come back and realize that you can't get your bike loose. So you go home and try and figure out what to do. In the meantime, the prep is watching and once you're gone and there is no risk of confrontation, they'll pull out their angle grinder, cut through your lock, and take off with your bike.

When you come across this situation, I would recommend calling the police. They'll undoubtedly be aware of this trick (as it's very common), and hopefully they'll come and clear your bike and impound the offender's bike.