I have noticed this behavior previously in a form based on a query where I later go back and make a modification to the query. It has now affected all of our applications when we upgraded to 24.2.3.
If you have columns defined in the database as NOT NULL and you use those columns in a form based on a query, Apex initially sets the VALUE REQUIRED property to YES (switch, switched). I suppose this is okay initially but it can be an issue after the fact.
If you make those items a checkbox and the VALUE REQUIRED property is set to Yes, then the only way you can submit the page and have it pass the validation that is implemented from that property is if the checkbox is checked. This is so even if a default value is defined in the database and a default value is specified on the item.
This is a BUG in Apex. VALUE REQUIRED does not make sense for a check box, if no unchecked value is specified, then the only way to pass would be to check the box. If an unchecked value is specified (and has a default) then it will always have a value.
This bug is easily duplicated.
Create a form from a query that includes a not null field.
Change the VALUE REQUIRED property of the field to no, save your changes.
Modify the query by adding another column. EVERY item based on a not null column gets reset back to VALUE REQUIRED = yes… this is an issue.