I got DVT last december from slip & fall on ice & got 2 blood clots in left calf. I was DVT free for 6 months & it returned in oct- why?

Might need testing. Dvts can occur after trauma. They can recur if you have estrogen in your body (birth control pills, etc) or are at risk for making clots. You might need testing like a hypercoagulation workup. Discuss with your doctor.
Hypercoagulable . Having a previous DVT is one the major risk factors to develop a recurrent one because DVT can cause chronic damage in the vein wall which predispose to DVT . Of course you need to be checked for hypercoagulable diseases because some of them need to treated with lifelong anticoagulation .