I would think that you were too fast - VNAV will only allow your speed to go 10kts faster than planned - if you (for any reason) are faster than that, it will disconnect (saying: "hey, pilot, I am in over my head. Do something for your salary and get me back on profile, then we can try again").
Once you are back at the (or close to) the planned speed, you can re-enable VNAV.
Jan