This is an old thread but in case someone has this issue I thought I would put in my two cents as I am going through this now. I was searching for anyone who might have seen some thing like this before and came across this thread. I installed a number of fan and toggle switches along with add-on switches in a number of ganged boxes. When complete one of the ARC fault breakers started tripping intermittently. I had seen this before when I has installed a fan switch in one of these ganged boxes a while back. There were two bundles of neutrals in the box and I assumed it made no difference which one I used. As soon as I powered on that circuit it would trip. I decided to try the other bundle of Neutrals and the problem went away.
The new switches I recently installed were in that same ganged box along with two additional boxes. Two breakers needed to be shut off to kill power to these boxes. In the box I had already added the fan switch I added two Add-on switches and the masters were in another box in another room. In that box that did not bring over the other neutral from the other circuit so I could not separate them. It would not trip every time but over the last month it has tripped about three times.
I opened up the main box and both circuits are on ARC fault breakers and the neutrals were tied together right next to each other in the box, so I didn’t see why I would need to separate them in the switch boxes. I thought about just switching it out with a regular breaker but decided to try replacing it to see if that made a difference. That seems to have fixed it. Can’t be 100% sure yet as I will have to see what happens over the next few weeks, but with the original breaker I had to turn the two circuits on in a certain order. If I flipped one on before the other the one I replaced would trip every time. It’s not doing that now. So maybe that was it. The breakers have been in since the house was built so they are all about 12 years old now.
Long story short if you see this it might just be the ARC fault breaker might be going bad.