What was Korean Air 672 doing at the time of Malaysia Air 370's disappearance?
With the way in which it suddenly switched back and forth it appears to be either a software glitch or bad data from one of the feeders.

All aircraft displayed in yellow on FlightRadar24 are detected by volunteers with their own ADS-B receivers, of varying quality in hardware, software and range. When aircraft reach the very edge of someone's receiver range you can occasionally get corrupted data, perhaps showing the aircraft somewhere back along its path or far off it. 

Another scenario is when a receiver is having trouble uploading its data to the FR24 servers because of a bad internet connection or similar, so position updates arrive later than they should.

Sometimes the FR24 servers get conflicting info, for the same reason, where one ADS-B feeder is providing good data and the other bad. When that happens you can see the plane seem to jump backwards and forwards until one of the receivers is out of range. 

It's something I've seen fairly often, even with my own receivers. FR24 does try to filter the anomalies out but it's a difficult task.