I think people can have several reasons to do these things.
For example they had a bad experience - attracted too many critiques or rude comments with the submitted image or something...
If someone had such experience they might consider removing the image.
Or they don't like the image any more, and feel it important to remove it.

And I think most people leave groups if they realize that they are really inactive in the group, and when they clean their group list, they leave the ones they don't use.
And of course there is the possibility of conflicts again...

My opinion is that group founders shouldn't take it too seriously, or personal unless several people start to do this in the same time, because that obviously means a problem with the group. But if only one member does this time to time, that's their business. If someone has a problem and needs help, they can contact the group admins.