My best guess is that some of your receivers were using a dodgy satellite, occasionally a satellite will loose sync with the rest of the constellation. They are closely monitored but it can take the control centre a while to command the rogue satellite offline.
This is one of the lesser known reasons why GPS should not be used as a primary means of navigation. There was a well reported incident about a year ago when one of the Russian Glonass satellites was causing errors of up to 10 miles for half a day, making life interesting for ships in the English Channel
and Dover Coastguard.