what am i doing wrong?

[Anubis_Nephthys]

While i agree that the other tank in this scenario should have solid aggro (so this SHOULDN'T ever happen), you can make up for another tank.
Perhaps the long animation/time to go off on RoH is the problem. In a tank swap scenario, it should always be on a tank once the fight has gotten underway.
Here's some tips that PLDs can use for snap aggro. (These are mostly useful for snap aggro on unclaimed/new mobs, but they still help.)
1.) Que up a Savage Blade instead of a RoH. Savage Blade goes off significantly faster than RoH. This way you get snap aggro very quickly, and you have a big daddy RoH right after.
2.) Use weave-ins on tank swaps. Spirits Within and CoS are my friends
3.) Perhaps even use Fight or Flight if you have to.
I hope you hadn't heard/thought of at least something here! Happy Tanking!