Step-by-step explanation:
To keep this short, the empire was so large it became near to impossible to control the borders. As the empire became bigger, tribes were joined into it. People could no longer be one with roman culture and weren't reliable enough to defend the borders. While the borders were getting more vast and the lack of people to guard it - they were attacked by different places. This led to being unaware of dangerous attacks and with that, they weren't able to keep up. In other words, too much land and too few people.