Well saying it's not the healers fault at all, without any info is kind of Misleading.

Was the healer overhealing excessively? we don't know
Was the healer way over geared compered to their tanks? we don't know
Did the healer start the fight with a HOT before pull? We don't know

There are also other reasons, but yes some tanks are having trouble adjusting, because they still want to push dps, even in content where it isn't needed.