In all my time dealing with ITP - 12 years - I've never seen physical trauma mentioned anywhere as a cause of it. A quick search I just now did turned up nothing, either... one article mentioned severe psychological trauma might sometimes effect platelet count the same as chronic, lower-level stress can.
The patient information section on this wonderful website has information about causes on this page -
pdsa.org/what-is-itp.html
Be sure to check the link within that information on "The science behind the causes of ITP", here -
pdsa.org/images/ScienceBehindCauses.pdf
It may comfort you to know that few people are ever able to identify the cause of their ITP. I hope your recent relapse isn't severe and can be safely monitored without needing treatment. How's your count now?