If he went in in 1909. I am sure he was healthy before he went into the Army. It sounds like something you would get from Battle fatigue. I know you said he didn't serve overseas and you don't have to say what or where.
If he was an ordinary, healthy and well balanced individual. It all sounds like he must have served in some capacity in a war zone. He could have been in security and was into some covert operations that they would not probably tell you about anyway. He probably took it to his grave.
The other side of the coin is that some people are not suited for military life and had a very bad time.It's like anything. If he was with a group of guys and he just didn't fit in or they didn't want him to fit in. The military can be a very cold and cruel place to. If they have it in for you. They can ride you and if you can't take it. You just might crack.
It's like that movie with Sir Alec Guiness. "Tunes of Glory". A lot of veterans will tell you that it had a lot of truth to it. Alec Guiness was the main culprit and between him and a few of the men they drove the new commanding officer (Sir John Mills) to the brink. It was a good movie and it is quite true in a way; but in the end Alec didn't fair so well either.
It's like working at a factory, shop, etc. You have to make a living and you have to go to work and pay the bills; but if you are not liked and they want to make your every day a living hell. They could do it then.
Sometimes people run away to find something. They think they have the sure fire answer; but they are really not cut out for it; but they thought it was the answer. They might leave there and go some place else and all the while what they really had in the first place was the best thing for them.
It's like being married. sometimes a person might get married two, three, or four times and keeps on getting a divorce or the marriage doesn't last. The simple truth is they keep marrying the same person all the time. The other side of the coin. They won't admit they have a situation with there own selves; but are too blind to see it.
I am not saying any of this applies to your relative. It is just some thoughts. You are seeking answers. I am trying to give some possible reasons. The answer you may never find. I truly hope you do. You don't have to answer this
God bless and God Speed,
Danny