It can happen so quickly: First, United Airlines had a massive pilot shortage and now, according to its own statements, it has significantly too many pilots. This is primarily due to the fact that Boeing cannot meet delivery commitments for the 737 Max orders.
In the meantime, United Airlines has stopped all new hiring. Now they are trying to reduce costs by offering pilots unpaid leave in May 2024 on a voluntary basis. The union assumes that it will not stay that way and that United Airlines may have to take further measures.