I'm having the same problem with my GM transmission. I've actually had my tranny rebuilt twice, thank you very much (ugh), so I've got quite a bit invested in keeping this car running (a 1999 328i). I've got the exact same symptoms you describe: spews some ATF when you put it in reverse after it's been allowed to cool down overnight, stops leaking after a bit of driving, and otherwise shifts really smoothly. The fluid that comes out is a pretty clean-looking cherry red, so at least I know I don't have shards of metal floating around in there.
My car is currently at the transmission shop. They put it up on the hoist to observe the leak and were able to reproduce it. They also noticed that it comes out more when giving it some gas.
The breather is mainly there to release excess air pressure as the fluid moves around inside the transmission. Otherwise, things might build up and go "boom"! (Bad news.) When it spews ATF instead, there is probably some kind of leak that is allowing the fluid to get pushed around where it shouldn't go, so it gets released the same way the air is supposed to be. The big question is where the leak is. I've been trying to find suggestions online for "usual suspects," but I haven't found anything. My guess is that it varies.
I'll post again if I find anything or after it's been fixed. I realize this message is kind of dated, but I didn't see a resolution, so I thought I'd post. I know I've let mine do this for quite a long time, driving it minimally. It's been in the tranny shop a couple of times at least. They keep doing things they think are fixing it (for free) and adding fluid back in, so I don't get too low. That's the main danger. Maybe you're doing the same.