Im actually from Split where the Torcida hooligans (Theyre football hooligans fans of "Hajduk" club") come from. I dont know about right now but one time a football hooligan faction in Belgrade did pay for some of our hooligans to go to Serbia and fight for that faction against another. But there were only about 10 people i think.
Right now i dont think thats whats happening.

1. The border between Croatia and Serbia is closed. (They could prolly find another way though)
2. This before was just a brawl between hooligans. Youd need 100s of Torcida members to create this level of riots. I dont think you can find that many. The number of Torcida extremists that are that crazy isnt that high, i think anyway.
3. Theyd have to stay in quarantine for 14 days on return and hooligans dont like staying indoors. They like to break shit and do drugs. :)