Originally Posted by Often1

UA uses fairly agressive software to locate "impossible" bookings and then backs out space which may have been manually protected by an agent a part of an IRROPS booking or the like.

Put simply, if it's likely impossible to fly all segments booked on a given day, the system may cancel one or both and may or may not notify you until it's too late to fix.

Better move here is to fix the issue in advance, inconvenient as it may be.