Some nameless official put anyone plausibly close to Snowden on one of however many "people of interest" type lists the UK has. It might even be a trickle down from some list in the USA. The original list may not even be intended as a "detain this person" list, but the way these secret lists work sourcing from other lists with different intents means the end result isn't necessarily the intended result.
Tyranny through bureaucracy basically. A little bit ironic what with the guy being Brazilian and Terry Gilliam's opus Brazil dealing with basically the same theme.