The short answer is probably "because you took too much insulin".
If you took as much insulin as you normally take for the same amount of food, there could be many reasons why you needed less last night.
The usual candidates for reduced insulin needs are exercise and hormonal changes (including menstrual cycle). However, weight loss as well as improved diet can increase insulin sensitivity, thereby reducing the amount of insulin you need.
Stress tends to increase the amount of insulin you need, so conversely, if your stress levels go down, you may find you need less insulin.
Finally, in newly diagnosed diabetics, the beta cells which produce insulin are usually "exhausted" because they have been overworked. When you improve your blood sugars they are given a rest and sometimes start producing more insulin than before. If your beta cells produce more insulin you need to inject less.