Distance X 300 times while running

While running outdoor on flat road.  Everything is working normal.  Up to about 28km, Enduro suddenly beeps and increase the distance by 1km and it continues to add displacement to the running.  I stopped at 34km.  Pressed stop and got the screen - Longest Run - 35582km.  This is the first time I turned on the 3D distance and 3D speed, any clue ?  BTW, although run on flat, total ascent was recorded as 14740m, total descent - 14730 and max elevation is 20,037,508m