WWE tricked Superstars into thinking they wouldn't be switching brands by asking them to stick around for "promotional work."
Some assume that WWE Superstars know what's in store for them weeks or months in advance. Truth be told, the people plotting out their careers don't know that information, so how could the Superstars? It seems far more common for wrestlers not to be told what the plan is until the last possible moment, sometimes not even until a few hours before a show.
Doing that in the present day actually makes a lot of sense. In a business with a focal point that is effectively a staged sport, stopping spoilers from getting out is an absolute nightmare. The more people who know what the results for an upcoming event are, the more chance there is of that information getting out into the world.
According to Wrestling Observer Newsletter, the switches that took place during this week's Superstar Shakeup were kept extremely under wraps. While that wouldn't have been a difficult thing to do on Raw, SmackDown Live would have proved to be more of a challenge. Asking Raw Superstars to stick around for SmackDown Live would all but reveal to them that they would be switching brands.
WWE's way around that was to ask Raw wrestlers to stick around for an extra day in order to perform some "promotional work." Then once they arrived on Tuesday, just hours before SmackDown Live, they were told that they would be calling Tuesday nights home for the foreseeable future. Some of the bigger stars were reportedly given more notice. We're assuming that includes the likes of Roman Reigns and possibly Finn Balor.
Chances are most Superstars don't like being left in the dark or surprised in this way, especially when the changes drastically affect their schedule. However, to avoid spoilers, WWE really doesn't have a choice. If wrestlers were told further in advance and that news got out somehow, the finger of blame would be pointed firmly at them. Keeping it a secret for as long as possible is the best thing to do, no matter how inconvenient it might be.