The United App will only show the most recent cause of the delay. There are often multiple factors that contribute to a delay and change. The new expended delay explanations help give more context.

Even though the app may only be showing the most recent cause of the delay, the entire record is still there and is used internally to track delays.

In a case where a flight is delayed due to waiting for an arriving aircraft that is actually running early or on time, something cause the delayed flight to be swapped into that aircraft but there's no way to tell, from just the app, if it was weather, maintenance, crew connection, or any number of other reasons.