It's not just crookedness that will cause the horse to stick it's tongue out, it can be from a number of factors. Dressagebelle, yes they can evade the bit by sticking the tongue out, but often you will see it accompanied by a twisted poll and tense back.
MaggiStar, tension is not really about work load being too much for a horse, it is more about how it is being ridden/trained. If the horse is stressed, anxious etc. that will create tension. If the rider is applying too much of an aid and the horse is quite sensitive, that will result in tension. If the rider is allowing the horse to plow around on the forehand, there will be tension in the back. If the horse is a 'leg mover' and does not swing the back, obviously there is going to be tension in the back. Most tension exists from the back or neck, so in most cases, once the back begins to swing and the horse comes 'through' to the bridle, the tension will ease.
If you are being too forceful with your rein aids, yes a horse may stick its tongue out to avoid the pressure. If your position is slightly one sided, the horse may compensate by twisting its body - becoming crooked, tilting at the poll and sticking its tongue out.
It is impossible to tell the cause of a tongue sticking out unless video is seen.