I believe it is a matter of territory. Place two males in a new clean enclosure and they will be less likely to fight, but place a stranger male into an established males enclosure, and forget about it. One of them will loose. There have been observations of sikorae scenting or marking their territory but data is lacking on the topic at the moment. This, however, does suggest they develop territories in which they feel the need to warn other animals of it. Or it could be to attract females, either way, it's still fascinating.
In my experience, males will eventually always wind up fighting. Even brothers of the same clutch who grown up together occasionally bicker and go after each other. The key is to have enough space for the weaker animal to get away, or the attacker may really do some damage.
Male sameiti seem to be the worst of the group with henkeli and fimbriatus coming in at a close second. sikroae males have never had issues for us, neither have lineatus males typically, but if you want to see a fun interaction, put a couple of guentheri males together for a while. Tons of barking and chirping, then they seem to dance around each other.
Note, while guentheri are not as visually striking (some would say) they definitely make up for it in personality!
It didn't take us very long to make a hard rule in our policy to not house multiple males together for the long term. When transporting etc, the stress is enough to let them forget about each other, but not as permanent residents.
Cheers!
Luis