That explains it then.mw, ours are very, very light and no anti-slip at the bottom. I just looked up what you were referring to, and it appears those are the ones we're using. They're yellow and so very light.
You could try some of the suggestions already made and/or add some anti-slip material to the bottom.
In the long run you should look for a quality 4 blade easel such as a Saunders.
As I already pointed out these can be found quite cheaply in 11x14 size and are well made, heavy and do not move around the baseboard without a lot of effort assuming the rubber on the bottom is intact.