It's the same as any other large organisation. Someone at the top says "oi, so-and-so has been a real pain lately huh". Someone one level down says "is there anything we can do about so-and-so?". Someone the next level down says "well, let's start a file on so-and-so, see if he's up to anything, no harm in that". And then since UK antiterrorism law is terribly written (or brilliantly written, depending on your perspective) such that essentially everyone with even a vague interest in politics breaks it all the time, and the surveillance state means the proof will be a couple of keystrokes away, once you look for a reason to go after someone you find one.

Probably no-one sat down and designed the system to work like this. But it works well for everyone involved, so...