Speedrunning: Status Buffs and Aggro Management.

[LionKing]

An opening rotation for a pull as Tank is
1/ Shield lob/Tomahawk to one mob in the group. Provoke even in some cases
2/ Flash/OP once the whole group get closed.

Based on the situation OP described, I imagine it was something like this:

Regen on the Tank. Tank went and used Tomahawk on one mob of the group, and waiting for the group to surround him as usual. However, thanks to Regen tick, they now went for the Healer and instead of running to the tank or stay near (my assumption, true or false,I don't know), he ran around and now the Tank could not use Flash or OP because the mobs were all scattered. Hence lead to the wipe because they killed the Healer.

Edit: Here is another situation:

Old Regen is ticking. The door was opened and the Tank went in and agroed the mobs with his pure presence only. Under normal circumstances, the mobs should go for the Tank solely as the DPS had yet opened fire nor the Healer was healing. However,because of the old regen ticked, they now chased after the Healer. Now we have two situation that could happen:

A/ The healer stayed close to the tank or run to the Tank and created an opportunity for the Tank to take them back by using Flash/OP and not use the big healing skills (Medica 2) until the Healer was clear of danger. Despite on that, and the tank failed. Then yes, he was a bad tank.
B/ The healer decided to play a game of Tom and Jerry and kitted the mobs around or run away from the tank (With Sprint or without Sprint) while the healer might or might not cast Regen on himself, causing the Tank had to chase after them and did his best to take things back but failed. In this case, we have a bad Healer as well.

In A scenario, the Tank will be fully taken the blame. In B scenario, the responsibility is shared between the Healer and the Tank.
The wipe could be avoidable if the Healer just stay close or run to the Tank when adds go after them. A good tank will know how to deal with such situation arises. However, they can not if any of the party members being stupid (running away or kite the mobs around) though.