I don't think parents "face 'behavior management' issues". I think the behavior management is something children, themselves, are responsible for.
The issue for parents is to know how to nurture, and talk to, children in a way that children generally manage their own behavior.
I guess I'd like it better if this question were worded, "What is the top 'behavior support' challenge faced by parents?" I don't know.. Somehow "behavior management" comes across as a little to controlling and/or removed.
Having put in my two cents on the term, "behavior management", I think the biggest challenge parents may face is finding a way to help kids be strong enough to resist outside influences, while also helping them be people who aren't outcasts among their peers.