A while back I blogged about this condition.
It's essentially what the name implies. There is no known single substrate nor is there a known single trigger (a number of patients seem to have had infection). Very few patients had known hereditary thrombophilia. Some had antipholpholipid syndrome and catastrophic APLS may be a subset of TS. It is defined clinically and likely represents a variety of (as yet undiscovered?) underlying disorders.
Despite a probable diversity of underlying hypercoagulable disorders TS is emerging as a clinically distinct entity. Since the above posting investigators have gotten more organized, and there are now more resources available regarding the condition which can be accessed here and here.