I never had this issue as a tank but have torn hate off tanks as a DPS.

Causes were often the following
1. large gap in gear level
2. tank not using tank stance
3. tank not using tools effectively
4. Excessive procs of crit + DH during boss fight opener.

One other observation to note is there are a lot of DPS players out there that seem to think they have to try and frontload everything they have as soon as possible on every pull, often taking the initial hate due to being the first person to actually engage the monster either by hitting it or getting too close and this seems to make the monster stick to them more than usual. If a DPS does this too much I don't bother pulling the enemies off them so they get it in their head that if they pull something they're going to tank it.