Here’s the initial statement from the RAIB gov.uk/…/collision-between-two-passenger-trains-n… - pirc_lover had it about right

One train developed a fault which caused it to stop. The second one, as seen on its dashcam, drove through a red signal (which it should have stopped at) and into the back of the first. They are still investigating why the second one didn’t stop at the signal and whether that might include brake failure, safety system failure, driver error, and/or any other factors.