I've never had much respect for John Reid, but even by his standards his sharp exit from government just as his policies are being implemented must be a new low.
When he signed on at the Home Office, he knew just fine what the political situation was. What's happened to change his mind ?
The only rational explanation I can see is that he knows he is doomed to fail and wants to get out to leave someone to try to rescue the situation, thus saving something of his reputation for a future leadership battle.
He should be a Man, have the courage of his convictions, and try to do the job he was appointed to. It won't be easy, there are many circumstances and people conspiring to make thing difficult, but a true statesman would persevere to the end.