What is the cause of small bumps on the heel?

You may be . Referring to piezogenic papules which are benign growths and are simply herniated fat.
Varies. This may be an inflammatory condition like athlete's foot, or what we can piezogenic pedal papules - lumps around the heel area from pressure. Fat lumps herniate from the network of tissue keeping the fat pad in place and can be the cause. Firm heel cups can often be the only treatment needed.