Why has my dog gone from very beta and pive to an aggressor that starts fights?

I have two 1.5 year old male siberian huskies. I am the one that typically cares for both of them (walks, food, brushing, cleaning, play, obedience, discipline) but I was out of town for 2 months and my livein boyfriend cared for them while I was gone. Since I have returned, they have had several fights (only one resulting in blood) and sometimes multiple fights a day. My slightly younger dog has been the one instigating the fights, though there has been provacation such as mounting by the other dog, but in the past this dog would always deter and never cause a fight. What has suddenly changed his behavior from very pive to very aggressive? I recently brought a new foster home(1 year old male siberian) and they are constantly fighting, which has never happened before. All of the dogs are neutered. I don't know if my absence has caused some problems or if my dog has just suddenly grown up. I need to curb this dominence over other dogs especially this new foster before I get hurt in their spats or they do.