I'm interested Jaydee, how you worked out ulcers hadn't been a factor with your horses? Do you mean they didn't suffer any digestive upset, such as acidosis or only ulcers were ruled out?
My knowledge about windsucking(or cribbing, as terms often used interchangeably) is largely academic. I have kept my horses with (paddocked)windsuckers for years & have seen stabled or yarded horses that were inadequately/unhealthily fed develop the problem, but other than that, I only know from studies I read/hear. It seems that it is always physical in cause, not from boredom or such, as people used to think. It's also not 'catching' & other horses don't copy the behaviour.
One study on Australian racehorses some years ago showed a huge percentage(pretty sure it was over 90% but don't quote me) of horses had ulcers and that the vast majority of those were windsuckers, and that no racehorse included in the study that was a non-windsucker had ulcers. Other studies I've seen say that acidosis causes discomfort(heartburn) which can cause horses to windsuck and also if ongoing/frequent tends to cause ulcers, among other problems.
I never trust my own judgement on anything that's affecting a horses ability to gain weight so if I buy a horse in poor condition I always get a Vet check to rule out anything that might not be fixable. Ulcers will always cause some degree of discomfort that will result in a horse reacting to pressure around the areas that are going to be affected, fractiousness from pain. Colicy attacks, anaemia from bleeding ulcers, digestive upsets etc. The last OTTB we bought that windsucked had a laparoscopy to rule them out as he did have fractious behaviour when we bought him - that was all down to handling issues and he mellowed very quickly when he had a normal life with us.
When my mare had them from medication she displayed all the outward signs though never began windsucking/cribbing. After treatment she was back to her normal relaxed happy self and soon regained condition
We bred 3 foals from the mare that windsucked and none of them ever did it or the other foals that ran in the same pasture. I bought a 6 month old foal - just weaned that began doing it as soon as the owners weaned him - by shutting him on his own in a small stable
Some horses handle life on a racing yard perfectly well due to temperament so the things that result from stress (windsucking/ulcers/weaving etc) are less likely to affect them so it wouldn't surprise me at all that a horse prone to ulcers will also windsuck though I'm not convinced that the two essentially have to go together or one causes the other
Windsucking becomes a habit so even when you remove all the stress factors and the horse is settled and relaxed they will still do it
It would be interesting to have some statistics as to when horses actually begin to do it