Today I was on UA1523 (MCO-ORD) departing at 0720. I was listening to CH9 and while we were en route in ATL Center, every a/c that was arriving into ATL was told to expect holding (weather), though not timeframe was given to them.

I"ve only been put into a hold pattern years ago when I was a kid, I remember the captain making the announcement.

I was curious as to how often this happens to other FT's, and why? I would guess weather & flow control being the major causes, any other reasons?

Thanks