irrelevant subject matters AND when to the wrong folder. but when it's submitted to the wrong folder, usually one of the admin "invites" the piece back in the right folder this time and leave a note for the artist who submitted
In the groups I run, I've honestly not had any reason to decline yet, BUT that may because of how super specific the groups are. One is for my husband and I's webcomic which accepts all art done of the characters and even fan characters. The other group is an OC Challenge that only accepts entries for each challenge.
in the art based group I vote on submissions for, the reason I vote no is usually lower quality or repetitive (I've seen the piece submitted before or it's the same char over and over again in similar poses. In the resources group I run, I vote no mostly because submissions are art not resources, on occasion I'll vote no when someone submits something that's almost the same as something that's in the gallery.
The most common reason is placement in the wrong category. In one of the groups where I am a Contributor there are too many submissions to move by hand. In one of the others, the submissions are so few that it's not a problem to hand-move them. In the third group I avoid the problem altogether by only have the "Featured" and a "Tutorials" category, because none of the members cared if the submissions were categorized.
The second most common reason (again, the Contributor group) is multiple submissions of the same item. I will usually accept one view and comment on one of the declined submissions suggesting thumbnails in the Comments section.
Thankfully, none of the groups I admin are particularly concerned with quality. Instead we have special categories for Tutorials, and encourage members to work to improve their skills.