It's because the forum smilie for
![](/static/smilies/biggrin.gif)
is : followed by D.
If your message is only capitals then the forum makes them small And Makes Your Sentence Come Up Like This.
If your message is only
![](/static/smilies/biggrin.gif)
, then the your post [according to the forum] is
:D
Therefore only capitals and therefore it makes the D into a d
![](/static/smilies/thumb.gif)