I wouldn't ask. It is none of your business why they pushed back the date. You have no reason or standing to ask that question, it will come of as weird or demanding.
I would act like for any other candidacy : you applied (through the channels they asked, I hope), they know you're interested. If they are interested in you, they will contact you when the time is right for them. They might never contact you. So continue your job search as if they won't, and let it be a nice surprise if and when they do.
You could ask what their timeline is like if you yourself are on a tight schedule (aka you have a good reason to disrupt them), but I wouldn't even do that. They didn't contact you yet, they didn't show their interest in you. So again, being asked by a candidate they didn't reach out to what their timeline is, could be seen as weird and/or entitled. If you're lucky it won't make a bad impression, but it won't make a good one either way.
Just imagine that every candidate who applies sends a follow up e-mail. It's a nightmare for the recruiter, and it helps nobody.