Multiple Checkbox gets locked randomly in IE9 above

I have page, there are multiple checkbox

checkbox locked randomly, means, I click on some checkbox, after that all checkbox locked. i am not able to check on checkbox. that is random behaviour. when browser is IE9 and above and doctype is XHTML.

but issue resolves when doctype is HTML5

so i am not able to figure out why this is happening. while this issue also not found in IE if version is 7 or 8.

any help will be highly appreciated.