Steam download issue

I'm not sure why Steam is doing that, but a solution would be to set the game download to high priority.

Right-click the game in your steam library, then go to properties --> updates, and set "automatic updates" to "high priority"

Another option would be to go to your downloads tab (view --> downloads) and pause all other updates.

Not all Steam games are hosted on the same server.

If you have multiple games going, and you pause one to try to download another, you may see different download speeds. This means that you're not making the same connection to get the game enough though it is still on Steam.

This means that if one server is particularly overloaded with a popular game, it may occasionally pause. If you have multiple games downloading, it'll simply move on to the next game.

Remove/pause your other games and let Steam download only this one game.