Perhaps an event needed foreshadowing, or consequences. Perhaps you've added a subplot.
And when you go back and look, you find that you actually wrote the plot in that section very tightly. Events stumbled one after the other because you had linked them well and they were dragged in by prior events as soon as they were done.
You get the same problem with dialog, when you need them to say something more, but you have one line of snappy dialog leading to another, and you have to pry it open and put in new, smooth, interesting segues from one topic to the next.