As I explained in a post sometime ago this day was going to come...

From the 30th of October 2011 our system will be being updated as part of the update to the V5 site system. Due to this system update: from late on the 29th of October you will be unable to manually edit your Minecraft IGN on the site, whatever Minecraft username you set from 10pm BST on the 29th will be attached to your forum account permanently.

I therefore am informing you all to ensure that you have got the correct Minecraft account, with correct spelling attached (no spaces either side, etc.) to your site account. It is up to you to check, once the system converts you will not be able to change this name.

I cannot stress enough how important it is that you have your valid Minecraft username as your IGN. The whole V5 experience for you will not work otherwise.

How to set your IGN on the forum
Profile > Modify Profile > Forum Profile > IGN (Simply enter your in-game username in the box next to that) > Save

If you have any queries/questions feel free to reply to this post.

why is this happening?

I'm pretty sure it's for the new points system? Because once you've donated and you've select items to by with your points, It automatically goes to your account in-game, So i think this is where it fits in - So it goes to your inventory straight away, without any fuss (But only if you've done it correct, I think this is what Lawrence meant when he said you wouldn't get the full v5 experience, If it isn't.