Mechanical issues and weather can happen at any time. It's reasonable to assume you could board a plane and then weather conditions change or a fault is identif

Mechanical issues and weather can happen at any time. It's reasonable to assume you could board a plane and then weather conditions change or a fault is identified. Deadheading crew is predictable and can be planned for. United should have been aware of the need for seats for those crew members and involuntarily denied boarding to enough passengers to accommodate them.