Police unions are a different kind of animal; not the type to campaign in solidarity with what is typically a Democratic political value.
In the Navy, we called a public sector union a 'mutiny'.
You'll be hard pressed to find a large police union that isn't Democratic, much like most unions.
At least on campaigns I've worked on they tend to endorse what comes out as the more 'conservative' candidate because it's all realpolitik: the candidate which promises the most police friendly policies and who has the best relation (and thus would be friendliest in office to) police, fire, ems, whatever group leaders.
Which are usually considered the more 'conservative' or policy friendly.