And it seems random; no rhyme or reason.
Is it each person's individual settings?
If I understand your question correctly, and you're talking about the width of the margins of an actual forum posting and not the editing window, then it most likely has to do with a large wide inline image in a posting knocking out those margin walls.
However, it appears the forum system admins may have very recently implemented an automatic image resizer function to keep this margin drift from occurring. The following originally 1200 x 1800 image has been automatically and proportionally "system resized" upon posting to "width=512 height=768". So, that whole
margins to the horizon thing may well be a thing of the past. But if you click the image, and you know you want to, you can see it uncorrected and watch those margins blast outwards. Click it again, and both she and the margins will spring back to "manageable"...
"I am in dire need of proper riding attire"