Whenever you use an item with durability, and the durability decreases, the items tooltip pops up even though you never switched items.

Ex. If I break a block of sand with a diamond pick, "Diamond Pickaxe" shows up above my hotbar even though I never switched items

If I fire an arrow, "Bow" shows up above my hotbar, again, I never switched items.

This does not happen in creative mode, as durability never decreases.