don't cops usually announce their name and the reason for their visit when they come to your house?

A real cop coming to your door has to identify themselves and show you I.D. They will always tell you why they are there. Be careful who you open the door to.

In the USA, it's your right to ask him these questions, but unless they were wearing body cams, and these cams were on, they could have reacted violently to your questions, and it would be your word against theirs. Their words being: "he attacked me".

they were probably just looking for someone who lived there before i did. they only asked me how long I had lived there and if I knew someone named nicholas

you have to ask for that stuff, you dont have to talk to him so you can ask him questions before answering anything.