From the UPS site:
"An exception occurs when a package or shipment encounters an unforeseen event, which could result in a change to the expected delivery day. Examples of exception include: address unknown, damage to shipment, or signature not received."
Basically an exception can mean anything. It could mean that it was loaded on the wrong truck (happens sometimes with no way to get it to the correct driver or to your address before midnight), it was stopped in security for some reason, or it needed to be repacked.
If you really want help with knowing the exact cause you would need to contact the UPS customer support and have your tracking # handy.