auto redirect to cart after 17 items are in the cart all of a sudden

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Like the title says, after adding 17+ items to the cart the page redirects to the cart, and I've had feedback from a couple of users that in some cases it's emptying the cart, but I haven't been able to repeat those results.

I haven't modified the product or cart pages in ages, and not sure why this is happening all of a sudden.

The site is denydesigns.com

Thanks!
Will Wojeck