I had this problem as well yesterday. It would start downloading, then immediately pause for no reason.
I after an annoying amount of time trouble shooting, I fixed the problem by disabling a program I have on my PC called "Game Booster," which clears background processes and increases processor performance.
My guess was that the increased performance was tricking Steam into thinking I was playing a game, that was causing it to pause the download. I'm not positive, but I do know that when I returned back to "normal" mode in Game Booster the problem went away. Oh and changing my DL location may have helped too.