If the wind isn't an issue, you're probably hooking the cast, ie, pulling the rod tip literally behind you on your back cast, so the line goes behind you, and when it comes forward, bang. It's quite common with heavy outfits as people put too much effort into the cast, use their shoulder, which then rolls around moving the rod tip too far around behind you in an arc.
No matter where the line goes in any cast, it's there because the rod tip took it there, this bit of logic can be used to diagnose numerous casting problems