Some might find Turner's comments unexpected, as auto sales have recovered during the past two years, and following the Great Recession, transactions reached an all-time high. Still, Turner suggested remaining cautious.

"It's taken an awful long time to get back to that level," he said.

And the numbers have not yet taken a dive, Turner said. Credit unions have seen a 32% increase in auto loans during the past two years. By comparison, during the same period, credit unions saw a 17% mortgage loan increase.

However, those figures can be deceiving, according to Turner. The majority of the loan increase can be attributed to larger credit unions – those with assets of $500 million or more.

This year may be the high point for auto sales, Paul Kirkbride, senior vice president of credit solutions at CU Direct, an Ontario, Calif.-based CUSO, said.

"This may be as good as it gets," Kirkbride said. "We could be at a peak. I think there will be a gradual drop off."

Turner pointed to several recent developments as evidence that his gloomy forecast is not simple pessimism. He said the national auto loan delinquency rate increased 6.4% in the fourth quarter of 2015, compared to the same period in 2014. When oil prices dropped, delinquency rates saw double digit increases.

In addition, he said, the percentage of loans with longer terms (72 months and longer) now encompasses 34% of sales. This is taking place in a low interest rate environment.

Drivers also are keeping their vehicles for longer periods of time, Turner said, citing a new IHS Automotive Survey that showed the typical car in the U.S. is at a record 11.5 years old and the number of vehicles that are at least 25 years old is about 14 million, up from eight million in 2002.

Adding to these trends is a fairly recent development – fewer people are driving. A recent study from the University of Michigan Transportation Research Institute showed 76.7% of people ages 20 to 24 had a driver's license in 2014, compared to 91.8% in 1983.

Kirkbride said one trend may help credit unions – the increase in auto leasing. Eventually, cars that currently are being leased will become certified used cars.

"Credit unions always have done well with used auto lending," Kirkbride said.

At least one credit union hasn't seen a drop in auto business. Summit Credit Union in Madison, Wis., had its best month in its auto lending history, according to Penny Armagost, the credit union's vice president of lending, analytics and credit risk.

Armagost said she too has heard predictions of a drop in auto loans, in addition to predictions of increases.

"I'm not really sure," she said when asked which prediction she believes. "We're going to plan that it's going to be a good year."

Armagost said one of the keys to ensuring a steady stream of auto loans is developing good relationships with dealers.

In addition, she said, officials from Summit, which has more than $2 billion in assets, have been discussing how to dig deeper into loans in an effort to determine why someone might be denied. "It's spending more time on loans," she said.

Kirkbride said when times are good and auto loans are increasing, it becomes easy for credit unions and other financial institutions to simply allow business to flow and not examine the lending process.

"When volume is good, you don't necessarily take a good look at your processes," Kirkbride said.

When business decreases, he said credit union executives should ask themselves, "Am I easy to work with?" as well as strive to do better than their competitors.

However, Armagost said her credit union will not just drop its loan standards.

"I don't think we want to take riskier loans simply to meet our goals," she said.