You need to play with the graph a little. You can zoom in a section by drawing a square or double click to jump to that spot in the table. Check other values in this area and plot them to see what happen just before. Playing it back is not always helpful.
Also you can click on the show on map and then click on the graph to have it create a point on the map where that was. For example maybe the GPS drifted at that moment or was going in the opposite direction from what you remember the flight did.