WHY?

I don't understand this dog anymore haha
everytime me and my husband go for a walk cobi is fine for the first 5 mins and sometimes we have a little run together where he just attacks maya. And its not play fighting, he really goes for her and its a struggle to have walks with them like that.
Is he jealous because maya is faster than him?
why is he doing this?
they go on superb with each other all the other times... Maybe they have sometimes a little argument but nothing serious.

When I walk them myself together they are fine but I don't have a run with them then so don't know If its that.