The way incursions were introduced in the universe was via being moving between universes. This created a bridge that connected the two universes and then if one universe doesn't destroy the other... they both get destroyed.

Doctor Strange and America Chavez teleport to a planet that has survived an incursion... but was also the cause of it. Instead of beating Thanos as the Avengers did... they beat him by Doctor Strange casting a spell (via the Darkhold). Which caused an incursion. Moving between universes can cause an incursion if the footprint left is large enough.

With what Strange, America and Witch were doing in the past, it wasn't large enough to cause an incursion. But what Witch intends to do at the end "stealing Chavez's powers and travel to a universe where she has kids" it would cause an incursion. Stopping Scarlet Witch isn't just to save Chavez... but the entire universe.

It's also the sort of really bad explanation of what will end up being the main plot point for the Kang movies. Kang is going to war with other universes and can preserve his reality by destroying their's. This gives impure motives for heroes who want to fight to keep their reality but also don't want to destroy someone else's.