Hey there,
Hard to say without seeing the form, but is the form one that you've published in the past and then updated with new required fields?
Initial thoughts are that some folks might have Version 1 of the form where some required fields weren't implemented yet. If the form was then updated to Version 2 which includes required fields, unless the folks that downloaded Version 1 update their forms to Version 2, they would still be utilizing Version 1 of the form and creating submissions without the required fields.
Not sure if that's your case, but food for thought.