In the series, Jumba mentioned various times of an ex-wife he despises so much for unknown reasons. She appeared for like 6 seconds in the Spooky episode (Halloween special).
What if in the future, there is an incident with some experiment Jumba created, and there is an antidote that might help save the day. But there's a problem, it was one of many things that was left behind at his ex-wife's house before he was kicked out. To Jumba's dismay, they will have to go back and face the one person he despises the most in order to retrieve it. Hopefully she didn't throw any of his things away, or they are all DOOMED!!!
Also, I remember people mentioning many times of how Jumba is in a way Stitch's father. For some reason I can imagine Stitch and Jumba feeling awkward, if ever someone brought up that subject. So clearly, with this reunion going on, Lilo thought this was the perfect opportunity to annoy Stitch, just for her amusement.
Lilo and Stitch (c)