Well I didn't realise this was something I did, or that it was strange until I read your post!
I definitely do it, there have been so many times when I'm with someone and really want to discuss something but just can't make myself do it, yet once I'm away from them I'm perfectly able to express myself clearly and with a lot less fear if I send it in a message.

I also find it easier to get my feelings across on here in writing than I would if I was saying it to someone in person. Who knows if it's a BPD thing or a general confidence thing with everyone in life though...