Given that Healers have the shortest queues by far, there really aren't lots more to replace them. Healers represent 25% of the group in most content but they are nowhere close to 25% of the overall player population. Given that Healers represent the main bottleneck in forming DF groups, any hit to their participation, even a minor one, has a disproportionate effect on queue times. Especially if those same Healers who are striking switch to other roles and add their numbers to the roles trying to get through the bottleneck.