Well I think I know why actually but just wondering if it's just best left alone. Our dc's are in the same class and often play out together BUT very often rub each other up the wrong way/bicker etc (so far so normal for ten year olds I guess). I've always viewed it as six of one half dozen of the other although I have taken exception to her dc giving mine light slaps around the face when they don't get their own way 
The falling out seemed to becoming a bit more regular and then there were two incidents whereby her ds made a point of excluding my ds. The first time he invited ds out for an activity (they both came and asked me if it was ok so I know he did ask ds) and when ds turned up in the morning he arrived to find his friend had changed his mind about inviting him and had left with his mum and another friend instead. DS was gutted and cried 
. Then a few weeks later they were all playing outside with a couple of other neighbourhood kids and this lad invited everyone except ds over to his house for milkshakes and told ds he couldn't come. DS said why couldn't they all just stay out and play together? His other friends agreed with him and this other lad stormed off alone (he flounces ALOT when he doesn't get his own way and I've seen this for myself as he's often done it when playing at our house) We've not seen or heard from him since and this was a few weeks ago now.
I told ds that sometimes people are just too different to get along well with and friendships sometimes run there course and that maybe he shouldn't actively seek out this boy anymore (they're both heading to different school anyway in September)
However they do attend the same club for an activity which has now restarted after summer break. We often swap lifts etc. Last night DH phone the dad to ask a question about the activity and the dad was very abrupt, gave one word answers and hung up without saying goodbye.
I haven't seen the mum but she is one of the kindest people I know so firstly the whole uninviting ds from an activity was a real surprise I can only assume she didn't know because it really wouldn't be like her to allow it.
If there has been something else going on that I don't know about between the boys I would have hoped she would come talk to me. I'm not a "my child would never be horrible type" and if something needed addressing I'd be on it.
Just not sure how to move forward given that we will see them weekly at this activity. Tempted to leave well alone but suspect DH will charge in like a bull and ask the dad "what's your problem mate?" Or say "well your sons a mardy sod so it's not all one sided " (DH doesn't do tact or diplomacy and neither does the dad, he's fairly gruff and outspoken)