Diversion of a plane carrying a head of state, to be precise.

But yeah, it's not easy to imagine how such decisions are made. Of course, it's one of the features of authoritarianism that the display of authority trumps "tactical mistakes". But it's people who give such orders, not "authoritarianism". The simplistic explanation would be to state they're "pissed off". But maybe reality is quite close to that. By arresting Greenwald's partner, the UK authorities primarily demonstrate, to themselves, that they still hold certain powers -- and I wouldn't underestimate the self-motivational dimension of such a demonstration.