Return of Reckoning

Would be interesting to see the kills score from the instance where you had the problem.
If you got stomped it might explain this as the enemy focus was working

Note: Most healing is done by properly rotating heal over time effects on right targets. You need to spam group heal only at crucial moment and to do so, you can use Focused Mind, unless you're WP, as WP has shorter cast times and Rampaging Siphon is much better in countering AoE burst damage and morale drops. Obvioulsy, don't forget about prehoting before fights.

are you sure it's set back, not interrupt?
set back and interrupt are two different things.
if u set back to 0 it's like interrupted.
in my case, set back to 0 is rare.
if that happen it's on my fault, slow reaction.

once u see casting set back more than 3 times in short interval, it's time to get out of there. and moving heal.

interrupt, not much you can do. it means you are being targeted. prepare inc and pre-kite.

I guess the answer is move to safer place to cast.