Sometimes I'll play a round and instead of 100 points or so I'll get like 1,000 plus whatever it is that I earned before. How did I do that? I've heard it happen to other people too.

I thought the same thing. I was just hoping it was actually something you could control. That would be awesome. XD

havent gotten 1000+ but got like 400+ points for playing invasion on spartan side yesterday, only thing i can think caused this was i helped return the bomb core, must have given a lot of points for sitting on it

the highest ive got is 2500, although i think its from those challanges for getting a certain amount of stickies or vehicles destroyed etc

Those are from commendations. You can see them in your stats at bungie. Sometimes during matches you get a headshot or stick and commendation pops up, thats when you fill the commendation bar. So lets just say first commendation bar is 20 headshots, you get the commendation during the game and it gives you 150 extra credits. Next commendation is 30 headshots, you get that and 300 credits added. And on and on, I don't know how high it goes. Higher commendations in areas show you what kind of player you are. Also in the full game there will be daily/weekly challenges for tons of credits, depending on difficulty.