It's unclear what the cause of Joy-Con drift is but one Joy-Con teardown published on Reddit by user u/rainbopython in April 2019 claims that the drift problem could be caused by wear-and-tear over time while other theories point towards build-up of dust inside the casing around the stick. We want our consumers to have fun with Nintendo Switch, and if anything falls short of this goal we always encourage them to visit so we can help.” Nintendo reacted with an official statement to the reports in July of 2019 and acknowledge the issue: 'We are aware of recent reports that some Joy-Con controllers are not responding correctly. Reports on Joy-Con drift led the filing of a class action lawsuit by the law firm of Chimicles Schwartz Kriner & Donaldson-Smith (CSK&D) against Nintendo in the United States District Court for the Western District of Washington. This problem was first reported early on in the Nintendo Switch's lifecycle. This means your character, or a reticle, could move in a direction without your input. Joy-Con drift is when Joy-Con register input where there is none.
What is Joy-Con drift and what causes it?