Best guess from here is that some transaction took a write-preventing lock on that table, and then sat on it for a really long time. If this were still going on, a look into pg_stat_activity and/or pg_locks would quickly prove or refute that theory. Since it's not, there's probably not much you can do right now unless you have complete query logs to study. What I'd suggest is familiarizing yourself with those views so you'll be better prepared to investigate next time.
Thanks for the response.
I have looked at pg_stat_activity previously but I had never heard of pg_locks before. Having said that, looking and knowing what to look for are quite different :-) I will spend some time trying to get to know both.
I am not sure what you mean by "complete query logs" but I do have a 1.3G Postgresql log file from the day in question. Is there a particular way that a "write-preventing lock" will be logged that I can search for in that file?
