This is incorrect. Trenchcutters spawn with aggro on tanks, not healers.
And since the healers are pretty well consistently healing at that point, they'll aggro them in no time, especially if it's a sage/sorc dropping their AoE. Consider that the tanks have no threat on any of the mobs to start aside from what they spawn with, and the healers are generating threat pretty well constantly..
That's actually part of why we guard the healers during this fight as well - especially since with the amount of tauntswapping you do on Kephess, no one ever really rips aggro for us, and the threat reduction helps somewhat.