The first thing that comes to mind is ripples in the fabric of space-time causing unexpected increases in velocity. The ripples would be something like gravity

The first thing that comes to mind is ripples in the fabric of space-time causing unexpected increases in velocity. The ripples would be something like gravity causing space to fold on itself in very tiny amounts causing a small "jump" in velocity for an object passing through such a folded ripple. Velocity would increase without an increase in acceleration. Yes I know, I watch too much Star Trek sometimes...