Several things can cause a horse to founder. Typically anything that can cause inflammation in the foot, restricting blood flow to the foot This damages the laminae (tissue that holds the foot and hoof wall together kind of like velcro)
Anything that brings on a fever, ingesting toxins, concussion (road founder) over-eating lush hay or pasture or grain, and drinking large amounts of water when over-heated. Retained placenta in the mare can also cause founder and using walnut shavings for bedding.
The only danger with feeding alfalfa is from suddenly changing from a grass hay to a rich legume. Alfalfa hay is fine if you make the switch gradually and do not over feed it. Mixing alfalfa with a grass hay is a good way to introduce alfalfa to your horse and makes a more ideal feed that straight alfalfa.
Here is a link that gives information on founder and what horses may be at risk.
I hope this helps answer your question. Please let me know if I can be of further assistance.
Donna C. Smith