Working/keeping busy/making others happy was how I coped for decades. 60 and 70 hour work weeks? No problem! Have a disaster you needed dealt with? I'm your girl! Family stress you need help to sort out? Yep I'll stay up all night and help you figure it out and then go to work on no sleep!
Some people do drugs, some drink, some stay busy.
Yes, yes, yes, yes, YES!
For me, it's not avoidance but rather a distraction technique and the more brain involvement it takes, the better it is with distracting me. Distraction isn't a bad thing. It can be but you need to be able to distract until you can deal with whatever you are distracting from.
Helping others many times helps the person feel better. It won't cure the mental illness/disorder/struggle they are dealing with but it does feel good for most to help those in need. So, that is likely part of it.
But there are many parts like layers in an onion. Maybe one helps others due to their trauma. It's not avoiding or distracting nor is it making them feel better. It is part of their trauma. Not sure what else to call that or what the function would be. I guess it depends on the trauma. But this is where I'd see someone doing for others to their detriment. Like never stopping and even when they know they will be triggered but still dragging on by their fingernails.
Example. I was forced to kill animals when I was growing up, which is a part of my trauma". As an adult I volunteered at a shelter and then got involved in volunteering with the local SPCA (Society for the Prevention of Cruelty to Animals. Many areas have local SPCA. Not all accepts volunteers. This one did) from a contact at the shelter. Those are the folks stopping dog fighting, closing down puppymills, responds to animal cruelity and the like. It was highly triggering and I worked off pure adrenaline mainly because I was seeing dying animals, animals with pieces of themselves ripped off, and the like. It made me feel better that these animals were saved from torture but it didn't make me feel better overall. It absolutely did not avoid or distract from my trauma. It was throwing trauma in my face everyday. Why did I do it then? Because I was forced to kill animals growing up and this was my way of atoning for that. I had and still have massive amounts of guilt there about killing animals. You can tell me that I was child and was forced to do it all day long, nothing makes that guilt go away. So, this was my way of relieving guilt and atoning for what I did. I did have to drag myself with my fingernails and I did, everytime.
That's just one example. You can put anything in it's place as long as it has to do with the person's trauma.
But, there are many volunteers that have no trauma in their past thus no PTSD. I don't think it's a sign of anything other then a kind heart and genorosity. I am not sure we should be looking to every aspect of a person's life for signs of anything. I mean, if it's costing you money that you don't have then ya'll should have a conversation. Maybe find something he can do volunteer wise in that same arra or topic (like for me it was with helping animals in need) that doesn't cost money. Just time. And understand that he may need to volunteer and that it has nothing to do with you or not wanting to spend time with you and the family but for whatever reason (be it needing to always be busy, having a distraction, or because of his trauma) it seems he may need this right now. Or it seems that way to me anyway. I, of course, could be wrong. But if it is costing you money that you don't have, you have all rights to put a boundry there. You should have a say in that.
Anyway, sorry this was so long but I hope it helps some to at least understand the different reasons one might need to volunteer.