Almost always because the map has been dropped into Free mode. Panning or zooming the map by hand does that deliberately, so you can look around without the map dragging itself back.

Tap the round follow button in the top-left corner to resume following. If the map had been rotated away from north, the first tap re-centres north and the next one resumes follow.

Each further tap steps through the follow modes — Bearing-Up, then 3D, then North-Up, then back to Bearing-Up. Bearing-Up rotates the map to your direction of travel; North-Up keeps north at the top. The map follows you in all of them.

If tapping the button does not help, the problem is the position rather than the mode. Check the GPS reading in the top status bar: it shows your fix precision in metres, and dashes when there is no usable fix. A cold start under tree cover, indoors or in a garage can take a while. Also check that DMD2 still has the location permission in Android settings.

More on the modes in Follow Modes.