While it's always difficult to predict the exact timing of when bubbles will burst, a combination of record-high lease returns in 2017 and 2018, combined with rising interest rates could imply that the auto bubble is on the precipice.

As Bloomberg recently pointed out, strong used car pricing is a critical component required to prop up the overall auto market.  While American's love their brand new cars, if used car prices become too soft then substitution can hurt new car sales.  Add to that the impact of falling residual values on the finance arms of the auto OEMs and you have all the ingredients required for an auto market meltdown.

Thanks in part to low interest rates, leasing has become an increasingly popular way to drive away a new car. It accounts for almost a third of all new car transactions in the U.S. and it's also huge in the U.K., as I explained here. For BMW and Mercedes-Benz in particular, it's been a boon for sales.

Typically a lease lasts about three years, after which the customer returns to the showroom for another vehicle -- which is when things could get difficult for the industry.