Most likely a friction material issue. Probably wet brakes, possibly worn ones. As mentioned, it's a warranty issue so have 'em take care of it if you have the time. Maybe try to keep a spare set onboard nextime (you'd have to pay for the set but they're really inexpensive) and learn to replace them yourself and adjust there rigger. It's pretty painless, and will save you the downtime, especially during your fishing freetime. Nothing more annoying than having your DRs out for warranty during a killer salmon bite or during the free time to take advantage of it. Seems to happen often to those that get water up under the lid of the electrics where it will eventually dampens the friction pads. If you travel to salmon fish with DRs, (ie wp/nb/lp/cq,) it's nice to have a spare set of friction pads just incase. It takes less than half an hour to swap them and re-adjust the downrigger.

Myles at Outdoor Emporium in Seattle is also factory authourized for Scotty warranty repair, and does excellent work with a quick turnaround.