[Walrus]

Before the answer, JNUS seems to be waiting to be enjoying the discussion and may be we still need some more discussion before hearing the answer.

Looking at it all over again and just giving one more thought - why can't it be Acute Subdural Hemorrhage instead?

The cause could be acceleration-deceleration force causing tear of the vessles. CT shows the mass to be somewhat crescentic shape if you notice the ends.