I can't think of a technical term, but "being at increased risk of falling" would be a non-technical version. "Falls prevention" (or fall prevention) is a recognised term for what to do about falls - there's even http://www.fallsprevention.co.uk/default.asp

I don't know if a search for falls prevention would get you anywhere?

"... there is loss of postural stability and a tendency to fall."
http://books.google.ch/books?id=mqg2dPYS-u0C...

" ... resulting in chronic dizziness, uncoordinated eye movements and a tendency to fall."
http://search.hopkinsmedicine.org/search?q=ca...

"I am concerned he does not appreciate the increased frailty, multiple disorders, polypharmacy, tendency to fall, and mild confusion so prevalent in elderly patients managed in primary care and the subsequent impact on the safety of anticoagulation."
http://www.bmj.com/cgi/content/full/317/7171/1502

"A tendency to fall over, either forwards or backwards"
http://healthed.aarphealthcare.com/ArticlePag...

why not simply a "tendency to fall (over)"?

Thx a lot...ok, sorry, I could have thought of that myself, but I'm still somehow assuming that if it's multiple words, and there's a preposition in it, it can't be a medical term - well, this might even be true in german...