This is something that happens with regularity. You were perhaps joining a queue that had grown quite long because, at that time of day, damage dealers were queueing up in much greater numbers than tanks or healers.
Imagine what the queue might look like: it’s 875 damage dealers, all listed in the order they joined the queue, and zero tanks or healers. With each tank+healer that come along, up to 3 of the players in the queue get a group and enter the encounter. At prime time, imagine that a tank+healer queues up once every 5 seconds, reducing the size of the queue by up to 36 damage dealers per minute.
Now imagine that a significant percentage of the tank+healer groups that queue already have one or two damage dealers in their group. They only need a fifth player. So only the person at the front of the line is removed from the queue and put into the encounter. In the meantime, the queue has grown from 875 to 885 damage dealers.
In this simplified** hypothetical, taking 36 damage dealers out of the queue per minute results in an estimate of 21 minutes to wait in a queue of 750 damage dealers.
** highly simplified, but hopefully workable for visualizing the situation