What causes a child to burp excessively at bedtime?

First thought : Kind of uncommon question... But the first thing i thought of is "aerophagia" or swallowing air. This usually happens from anxiety, so the fact that u mentioned "bed time" is interesting (kids often get anxious). A lot more information would be needed to go thru the other possibilities. If your child being anxious makes sense then that's it. Talk to a pediatrician to see what might be the issue.