Crossfiring is usually a lazy behavior, especially if a horse does it both ways. When a horse only does it one way, it can be a horse that is stiff or sore in its back, but is still usually a horse that is resistant and pulling on the longe line that direction or leaning and pulling on the inside rein when being ridden in a circle that direction.
It is very typical for a horse to 'drift out' and crossfire in the process when they go past the gate or past an area that they would rather be in. If this is the case, you just have a horse using crossfiring as part of their disobedience and resistance to your rein and leg aids. The horse is definitely NOT staying 'between your reins' and 'between your legs'.
These horses quit crossfiring when the rider pulls less on the bridle reins and pushes harder and more firmly with his/her legs. It usually means the horse has not been taught to yield properly to the rider's outside leg.
Horses have to be taught to properly yield to the riders 'lateral aids' first. Lateral aids are those that combine the left rein with the left leg. [Think leg yielding exercises, turns on the forehand and side-passing.] These are the easy ones. The opposite leg and rein only support while the dominant aids are the rein and leg on the same side.
It is far more difficult to teach proper response to the 'diagonal' aids. This is when the dominant aids are one rein and the opposite leg while the other rein and leg only support. These are the aids used in teaching things like good lead departures, the turn on the haunches, flying lead changes and the proper half-pass. These are very advanced compared to the lateral aids.
A horse that crossfires is 'flopping' its butt out as a sign of resistance and does not respond well enough to the rider's outside leg to fix it.
It is a really bad habit to let a horse fall into. It is a lazy, sloppy way for a horse to travel and usually indicates a horse that is heavy on its forehand and has zero collection. If this describes your horse, then it is just a lazy and bad habit and not pain or soreness related.
This can also start if a horse gains a lot of weight and gets fat and stiff --- and lazy.
I, personally, never let a horse crossfire just so they will not get comfortable with it. I always get after them hard and chase them around harder and harder until they bring that inside hind foot up and get both ends in the inside lead.
I never let them slow down to get correct -- I always make them go forward faster until they get it right. I do the same thing under saddle. They will get sloppyer and sloppyer and slow down and break stride if you do it any other way. Every horse I have will catch the inside hind leg back up if I just yell at them because I have tried to eat them up whenever they let that hind end fall out.
Letting a horse drag a back lead is kind of the epitome of anti-collection. If you let a horse do it for a long time, I guarantee that horse does not want any part of collection and of properly wanting to use its hind end.
At this point, I would bit this horse up with a snaffle and a surscingle and force it into proper frame and start longeing all over again -- both on a line and in a round pen -- and teach this horse to properly use its hind end.
Thanks for all the info, it is going to be very helpful. When I went back to the barn I ran my fingers down his spine and he didn't show any sign of discomfort or pain so I am going to go with that's not it. Actually he has had a sore back once before and he was off of work for a couple of days because of it so, that's definitely not it.
Sorry, just trying to get some details out here.
He's definitely not fat but, he is a lazy boy. I have to squeeze and squeeze and squeeze for him to extend and work into the bit, which we are also working on and he's been doing well. Right now he's in a snaffle bit 5'. He's been in a rubber snaffle too before and he liked that bit and wasn't pulling on me at all so I am thinking of switching him back to the rubber.
He pulls on me a lot and it's actually left marks on my hand(i don't have the word for it) we are working on collection and I have been working on my leg aids with him, figure 8's (he hates them) but, he doesn't do it in both directions I have realized he only does it in the one direction.
I do need to get a lot stricter with him and I know I need to get a lot stricter with my leg aids because, sometimes I am eh and let it go. (i have gotten better with my riding, i must get a video) He does do it while tracking left but, won't do it while tracking right. He is stiff a little bit tracking left but, he's sound (as said via vet).
We have done a flexion test on him and he trots away fine, each leg lifted and he trots away fine without any limping or anything. So, I don't know. I am going to have a chiro come out for him and still have the vet take a look at him.