I've had symptoms of ocd since my traumatic brain injury why is this?

OCD is an uncommon, : but well-documented, consequence of TBI. The pattern of OCD, I.e., the types of obsessions & compulsions, are very similar amongst patients with TBI, whether they have mild TBI & normal MRI's or moderate & severe TBI with bruising of the the fronto-temporal cortex, subcortical structures (caudate nucleus) or both on their MRI's. Just one more thing for which you need therapy &, maybe, medication.