That's because dumbbells allow you to increase the range of motion on all exercises and they are very helpful when it comes to achieving maximal contraction of the muscles. (fitnessandpower.com)
Unstable surfaces appear not to affect latissimus dorsi muscle activity, while smaller shoulder joint angles (regardless of plane) appear to display greater muscle activity during maximal contractions. (strengthandconditioningresearch.com)
To prevent this from happening, focus on achieving maximal contraction instead of allowing momentum to do the work and chose a weight that will allow you to perform the full range of motion in a slow and controlled manner. (fitnessandpower.com)