Part of this slowing down process is simply to do with your level of enthusiasm. When you start off you've got heaps of it. You tweet constantly, and because the more you tweet the more followers you get, you remain motivated to keep going at the same blistering pace.
Also, when you're starting off you can follow others in the hope they'll do the same in return. Such people are easy to find so you follow as many as you can. Consequently your follower numbers go up pretty quickly. But then you hit that 2000 limit and you can't do this anymore. So the rate at which people follow you drops substantially. And because a lot of those people were only following you so you'd reciprocate, many of them unfollow you eventually themselves. That certainly slows down the rate at which your network continues to grow.
You can then fall into another trap: tweeting just for the sake of it so as to keep your daily tweet quota up. If you do this the quality of your tweets will go down. Some of those who began following you primarily for the quality of your tweets may be disappointed and stop following you.
Because both these processes are at work it's really important to follow others primarily because they look genuine and interesting, and also to tweet as much good quality content as possible. Then your following will build slowly but inexorably. Those people will be more likely to stick with you, too.