All the bosses have enrage timers that are very difficult to beat for a first tier raid.
Are you talking about hardmode/nightmare ops? Because our ops group haven't seen an enrage besides the one on Soa and even there it was because we slacked some time switching targets from mindtraps to the boss and back again. And we did all the bosses on normal with two tanks and dps that were in a very average gear to say the least - no set bonuses, no lvl 56 pve gear.
Also, on the pylons fight theres actually alot of damage to the tanks when two elites pop out. Probably it's doable if you CC one elite while killing another, but I dont see why you wouldn't bring a second tank on this encounter, especially on hard/nightmare. But as to the other bosses in EV - yeah, only one tank is needed.