Did you look closely at the old gasket? If it's not abraded or torn, the only thing I can think of is there was a piece of crud under the gasket and when you removed the old filter it got cleaned off. It would be interesting to cut that filter open and see if the filter element was clogged. If so, the pressure could have been rising enough to open the bypass and that was enough pressure to force some oil out around the gasket.
