See a lot of people saying how "The gnasher is OP, that's why everyone swaps to it at spawn" and other various things along those lines.
While I can't speak for everyone (obviously..) I think it's safe to say a lot of people, like me, switch because it's safer.
If you're spawning, it means you're going to have to run across the map to get to wherever you're going. At any point in time, someone could run round a corner, or from behind you, and if you've got your rifle out, chances are you will die (unless you have the retro, in which case enjoy your free kill).
If you have the gnasher out, you can defend yourself and potentially win. You've a much higher chance of being forced into CQC when running across the map, so it makes sense to keep your gnasher out.