Why will an accelerated electron (or a charge) emit a photon?
For example normal matter even if it accelerate or not will emit a photon.
Any charged particle creates an electric field. If that particle accelerates then the electric field changes. This change in the electric field moves through space at the speed of light and we call this change, or disturbance, a photon.
There is MUCH more detail than I gave, but given your question I think you'll find the answer suitable. Leave a comment if you have any follow-up questions.