Well my guess here is that the tanks the OP are talking about are looking for "in progress" runs and actually only want to join "in progress" and not from the very start of the run. This is a very popular thing in say AK where the only thing many people need is the myth from last boss, and with the high number of failures at the wall, it is easy to join at the wall (and sometimes after which is just a bonus only needing to kill the last boss).

Now if this same tank (you don't know could be multiple looking for in progress) are instantly queue, withdraw, and re-queue over and over for 15min, hes likely just trolling and their is an issue. I do look for in progress, but rarely do i try to re-queue more than twice and then its a few min before I try again so I don't do what they OP is describing, but apparently other do.

Sadly, add a penalty for not accepting and less and less people will join in progress runs and when someone leaves your waits will be much longer