Complex sentence: Goals scored by goalkeepers are a somewhat rare event in football, for the reason that if a goalkeeper leaves the goal, the goal is left unguarded from long-distance attempts until the goalkeeper can return to defend it.
Split into simpler sentences: Goals scored by goalkeepers are a somewhat rare event in football. It is highly unusual for a goalkeeper to move far beyond this area and join an attack, as this leaves the defence vulnerable to long-distance attempts until the goalkeeper can return to defend it.