I spoke with another high ranking police official about the Gates case. He is pretty candid with me about most things, so I believe what he had to say. Here were his thoughts:
1) Disorderly conduct is an easy trap to fall into. When dealing with an officer who gets out of line, you should cooperate and then deal with the situation later. If Gates argued with the officer and yelled at him from the porch, that opened the door for him to be arrested.
2) If there were extra officers at the scene, it's likely that they were called when the officer arrived and realized that there were two men in the house (Gates and the driver) and only one of him. It's standard procedure to call for additional backup when you are outnumbered. But then again, I am not sure if the driver had left by then or not.
3) There are usually extraneous variables that have to be checked out in these situations. For example, even though Gates showed that he owned the home, he could have had a restraining order against him filed by his wife in the middle of a nasty divorce. Given that Gates had appeared to be breaking into the house (by pushing the door), the officer would be expected to make sure that Gates was not there to hurt his wife or do something illegal. Of course this sounds absurd in this case, but there are many cases where husbands break into the homes of their estranged wives in order to hurt them.
Just thought I would share these facts. The conversation was very interesting and insightful.