To reproduce, obtain a diamond pickaxe and begin mining. Every time you mine a block, the pickaxe will "pulse" in the hotbar – the same pulsing done when an item is collected which adds to a stack. This also affects attacking mobs using a sword, or otherwise reducing the durability of an item.

An item enchanted with unbreaking, for instance, will not always pulse, further signifying this pulsing coincides with the durability decreasing.