two reasons come into my mind:
1. you start to relax when you're on the winning side and don't calculate or think about your position as much as when it was even
2. you have an advantage and try to force the win, forget your opponents plans and moves and so on.
Concentrate on the whole position. You can develop and execute your plans, but look where your opponent can get counter-play.
TheProfessor I looked at your recent games and saw only one loss which I think still had quite a bit of play within so it's somewhat difficult to offer a good explanation to your inquiry. Although there are a lot of good suggestions here, I think Kageri's comment was on point. Chess is not a game where when you have the advantage it is beneficial to charge in recklessly. Trading off pieces when you have an advantage can be beneficial, especially when your adversary is a very good player; however you may feel more satisfied with your victories in which you manage to retain most of your material. In chess hesitation and calculation even when winning is a good course to follow, that's my opinion anyway.
It's human to relax a bit when you have an advantage--don't. There are always resources in the position for a resourceful opponent. It aint over, until it's really over.