Some reasons can be avoided, others can be a surprise.
The company took its lessons from exceptionally high mileage of hundreds of thousands of miles per car, operating 8 out of 10 of the highest mileage Teslas in the world. Some of the cars - like the Tesla Model X 90D “Deuxy” are doing great, covering over 350,000 miles (563,150 km) on an original battery and drive units, with only 13% of estimated capacity/range fade.
One of the first reasons that appeared early was when the cars were used in their full available state-of-charge window from almost 0% to 100% of charge, and using DC fast charging as a standard method every day. In a few months, one of the first cars lost 20 miles of range (6%), which was more than expected. The practice of using the cars changed to charge only up to 95%, and not discharge too near 0% if possible. As a result, battery degradation can be kept under control.
The other issue in one of the cars was pretty rare (few in the world) - car shut down, despite still having 50 miles of range. Tesla decided to replace the battery with a new one and said that the problem was in battery assembly, which in fact doesn't explain much.
The Tesla Model 3 is yet another story, as the car shut down despite it having left around 60 miles of range and no action was taken. This time the problem was in the touchscreen... as the "phantom touch" prevented the turning off of the screen, air conditioning, accessories and thus it drained the battery overnight. The touchscreen was then replaced by the manufacturer.
All those examples are completely different from others that could appear - hardware, software, temperature, charging behavior and more. This is why Haydn Sonnad, Tesloop founder, encourages all Tesla owners to sign in for another project, the Carmiq Connect to collect and analyze Tesla car data.