My boyfriend used to suffer this quite a lot. For no apparent reason one eye would just go bright red for a few days. There was never a common factor, it just happened. He had several visits to specialist who all just gave blank expressions and shrugged their shoulders. Until the last one he saw diagnosed it as a form of arthritis in his eye. He gave him some drops that made him go blind for the day but that was only a short term solution, there is nothing that can be done long term.