The trigger for this sudden change? The filing also contains a newly disclosed dispute with Rolls‑Royce involving a widebody engine purchase and maintenance agreement, undoubtedly referring to the Trent XWB engines that would equip any future A350s. The dispute centers around a $175 million commitment payment, and a demand for repayment after what United calls a breach, the kind of dispute that can easily ripple into fleet plans when an aircraft has a single engine supplier.