I think I may be getting to the bottom of what may have happened here. It looks like these are all symptoms of something having happened with the conductor panels. On 387s, the conductor panels have the usual Electrostar door close / interlock, signal bell, and "dim lights" buttons. An electrical fault with one or multiple panels could need the driver to override a conductor's key being incorrectly detected (a "phantom" detection), resulting more than one door close being initiated at specific doors. In addition, and this is something I have seen in practice, a fault with a panel can mean that the lights enter a dimmed setting without being commanded to do so. IIRC this lighting error can only happen if the train thinks the panel is active. Conductor panels do become active from time to time without this being commanded. Whilst the panel wiring on Electrostars is less complicated than on Turbostars (the functionality of which can really be quite "interesting"), it is still quite complicated. To give a handful of irrelevant but hopefully illuminating examples, the CACUs (Combined Audio Communications Units) in the cabs combine with the panel handsets to create a very useful internal telephone network, then there's the interaction of the PIS, the wiring of the key switch and the bell controls being linked, etc. etc. etc. In particular, if anything goes wrong with the key switch or any sort of failsafe device, it can have some very strange effects on panel performance.
(I once ended up on a 12 coach 377 which had unusually dimmed lighting despite nobody having asked the train to do that - it turned out that one of the 3 units somehow needed the "dim lights" button to be reset at one of the panels. After a lot of trial and error with different MCBs and buttons, the problem was eventually resolved!)
So it may well not have been anything too sinister, but I should imagine it was very annoying for the driver and would have been reported to Fleet ASAP!