Cat tipping over litter boxes?

I know this may sound strange, but I believe that Echo is tipping over the litter boxes. I have 3 boxes upstairs (the loft) and in the past 2 days, 2 of them have been tipped over. The first time I seen that it was her...she was trying to jump from one litter box to the other (they are about 2 or 3 feet apart). She somehow didn't make it over the edge of the box she was in and it tipped over & had litter ALL OVER THE PLACE! I just kind of dismissed it as an accident until last night. The other box that is in the opposite side of the loft was completely turned over. If that wasn't a mess! Thank God I just changed the litter or else I'd have a bigger mess on my hands. I'm just wondering how to prevent the boxes from tipping over. I'm kind of at a loss on what to do. Also, does anyone know why she is knocking them over? I guess I'm just assuming that it is Echo since I seen her do it first...but I guess it could be Bud or Tiki too. Any suggestions would be great! Thanks!
-Shell

Shell just put the litter boxes inside a bigger cardboard box. Cut holes in the box's sides so the cat can go in and out. Put a piece of astro turf down underneath to catch the remaining litter that spills out and problem is solved.

Thanks! I'll give it a try and it how it works. But...it brings me to this question...What if the cats don't like going in a box that is covered? I've never had a box that wasn't completely open all the way around...so I don't know how they are going to handle that. I'll give it a try though...it won't hurt to try at least. If they don't like it, do you have any other ideas? I'll let you know if it works out...Thanks a bunch Mary Anne! I know I can always count on you for some great suggestions!
-Shell

The box is open- flip the box over, cut off the flaps on the ends and the side of one end of the box. Place the litter pan inside the cardboard box with nothing covering it. Many of the cats will just end up hopping in and out of the box without using the precut doors.

Jeez...Now I feel kinda dumb! I guess I was just picturing it in my mind that the box would be over the litter box..not under it! Thanks once again! I'll have to get a few boxes from work tomorrow, so I'll post if I've got any more crazy questions about this.
Thanks,
Shell