It was a calculated risk.  Support the BC Liberals and face a major backlash from supporters much like Nick Clegg of the LibDems did in the UK when he propped up the Cameron government or support the NDP and hope PR gets implemented before the next election.  No immediate backlash from supporters, but will probably cost the Greens votes since people like what the NDP does, they will just vote for the real deal and if they don't like what they do the Greens will get punished for enabling them.