In Step Up (film series), Alyson Stoner only appeared in the 1st film, then 3rd film, and now she is confirmed for 5th film. In some speculation I have heard that she is only going to appear in odd numbered sequels of the film series even before release of the 4th film and which proved right because she was not in 4th sequel but coming in 5th film of the franchise, making her third entry again in an odd numbered sequel. Is this just a coincident or is there something else behind it?
I'm going on speculation here, but I think it's more chance than anything else. Around the time of Step Up 2, she was busy starring (in a better role than the one she had in Step Up) in Camp Rock. This is an interview with her:
Firstly, the italicized portion strongly suggests it was her own choice not to star in the film, as opposed to the director/casting team. Secondly, it doesn't seem to suggest there is any bizarre reason behind this. It seems very logical that she simply had other projects to do and wanted to do them, but was excited at the chance to go back for Step Up 3.
As for Step Up 4, I'm not sure of the exact reason. I've read that she started college at the time of filming and her schedule couldn't handle both. I also know she had her heart set on auditioning for the role of Katniss in The Hunger Games. Perhaps disappointment from not getting that role turned her away from filming for a short while?
Obviously there's a lot of speculation here, but I just wanted to point out that there doesn't seem to be any superstitious or otherwise bizarre reason for her appearances in only odd-numbered Step Up films. It just seems to be chance based on her life situation at the time.