Looking at the logs I do see where the calculated position and the GPS position diverge. They stayed that way for quite some time. This causes the EKF error as the two position do not agree.
It looks like the copter flew North though. Did not see were it went south at all. It looks like the copter flew to your Left as you were looking at it.
At the time of the incident I can see the GPS had some issues. The HDOP climbed very high and the number of Satellites dropped to 8. It looks like a GPS issue. It seemed to clear right after that.