Contractile roots

Anybody know of any agave species with contractile roots?

I have an outdoor bed with several agave species (also dyckias, 
aloes, pachypodium, etc) and one in particular seems to be pulling 
itself down into the soil; it is an unidentified plant I found at a 
chain store, and none of the other plants are doing this.  
Interesting thing is that this soil is a fairly hard clay and you 
couldn't push a plant down into it even if you wanted to. 

The plants have been there for about a year, and I first noticed this 
effect after this past winter, maybe two or three months ago. 
I really need to get a camera to show how it looks...

Regards,
Peyton Walton
southern Louisiana
USDA zone 9