Unity Bug? Parent object rotates around child object.

Hi all. I'm having a big problem with Unity at the moment. It only just started and everything was working fine before, which is why I think it's a bug. Basically, when I move/create an object into an existing object, that existing object will now rotate around the new object.

For example, if there's an object called objecta and objectb, when I move objectb into objecta, objecta now rotates around the origin point of objectb if that makes sense. Really have no idea how this has happened as it's never happened before. I've tried restarting Unity multiple times, restarting my PC. If anyone knows how to solve this issue that will be great (maybe I just enabled something by accident.) Thanks anyway.