One possible explanation is that you had a simple let down. This is a natural response to the stress experienced moments ago, and your organism is just trying to relax and regain energy.
But one of the symptoms or consequences of doing that is that you started playing the score. You see that you have a big lead then you start:
- Experimenting with shots and tactics
- Trying to show off to spectators
- Trying to toy with your opponent so as to demonstrate your superiority (ego)
- Trying to practice some tactical or technical elements
So forget about having a lead and start relaxing.
Ask yourself this question: Do those options--experimenting, displaying superiority, practicing, showing off--increase or decrease your chances of winning the match?
Because that's why we are playing a match, right? To win it.
Yet we always want more. We want to win comfortably, to demonstrate our superiority. And at the same time experiment and show off how good we are.
Doing any of that is asking for trouble. You might get away with it once or twice a year, but the rest of the time you just complicate things.
There are two approaches to this problem.
One is to simply play the point. Forget the score, play point by point, and try to win every point by giving 100 percent. This is the approach of great players like Monica Seles, Rafael Nadal and others. They fight for every ball, regardless of the score, and give their best all the time.
There is another approach, but it requires lots of experience, good judgment of the abilities and mental qualities of both you and your opponent. It also requires the readiness to accept that sometimes you won't choose the right method at the right time.
This approach is playing the score but in a masterful way.
Here are some examples of how top players use playing the score at the right moment: