From the CFPB:

Today, we released a report that describes complaints we received related to the private student loan industry’s practice of placing borrowers in default even when their loans are current and in good standing. We’re also warning consumers that they can avoid surprise defaults by pursuing a co-signer release.

The vast majority of private student loans today have a co-signer (typically a parent or a grandparent). Having a co-signer can often lead to a lower interest rate, which can save you money in the long-term, because the co-signer will have to repay the loan if you don’t.

However, your loan might also contain provisions that allow your student loan servicer to put you in default — even if you’ve been making your payments on time.

That’s because your co-signer is also on the hook for your loan and therefore changes in their behavior can impact your loan, causing your loan to default and making your entire balance due all at once. We’ve received complaints that private student loan servicers are placing borrowers into default when their co-signer dies or files for bankruptcy.

Read more at: http://www.consumerfinance.gov/blog/consumer-advisory-co-signers-can-cause-surprise-defaults-on-your-private-student-loans/