Spinner doesn't show up for itself: it simply means: "box busy in other operations".
So, as Matrix said, it might be some plugin or, I add, an attempt to use a connected device, like a NAS, a USB stick or the internal HDD itself.
If, for example, the internal HDD becomes faulty and needs more attempts to read a sector, you'll surely have an impasse and the box gets stuck or slows down.
So, the spinner lets you know this fact.