Is this a common thing for tanks to do?

[Adire]

Doing large pulls that is, and missing aoe aggro on some mobs.

The other day while I was healing, a tank was running and missed several mobs with flash. While running, their HP was getting low, so obviously I had to toss a heal on them.

When I did, the mobs doubled back for me and got in enough hits to interrupt my casts a few times, and they dropped us both to low health.

When the tank suddenly got aggro on them after several seconds, the added strength of the mobs that were attacking me finished him off while I was trying to heal myself because one more round of attacks would have killed me, but the tank was further away from dying from the mobs attacking him. He also got vulnerability up on himself from the vines (AK HM).

He raged at me as if it was my fault we died.

Why do some tanks not think it a good idea to ensure every mob they're pulling has been hit, even if it means stopping for a second before continuing to the next group? I never have this problem when tanks do that....a tank stopping for a second may take a few more hits for couple of seconds, but at least then I can heal them if necessary.