The Marvel Cinematic Universe is known for its interconnectivity, with major characters making unexpected appearances in various movies.
This trend was set early on, with Robert Downey Jr. appearing in The Incredible Hulk, and continued with recurring appearances by Samuel L. Jackson and Tom Hiddleston as Loki.
Villains have also made surprise returns, such as Samuel Sterns in Captain America: Brave New World 17 years after his last appearance.
The studio made this clear when Robert Downey Jr. appeared unannounced in The Incredible Hulk
Author's summary: Ultron's MCU return may ruin the Avengers villain.