The goggles are leaking
If the fit of your goggles is not quite right, water may be able to enter the lens. Adjusting the fit or changing the nose bridge should solve the issue.
The strap should hold the goggles firmly enough in place so that they do not slip during turns, but not so tight that they cause discomfort. The strap tension is adjustable with the buckle at the center of the strap. Adjust the strap one side at a time by releasing the end and pulling it through the buckle. Always fix the end of the strap back down to prevent slipping during use.
If the goggles are leaking during use, and tension adjustments will not solve the problem, switching the nose bridge should help.
You can find more information about switching the nose bridges and adjusting the strap here.