You got that backwards! Almost all trades are conditional on the physical so the trade doesn't become official until after the physical.
But trades can't be executed until the start of the new league year (today).
EDITED TO ADD:
Now I'm getting more info. There was no actual physical performed. Supposedly the Cowboys team doctor who performed surgeries on Patrick Mahomes and Micah Parsons, reviewed the images of Crosby's knee and gave a less than optimistic report. The Ravens read that report and that's what scared them into backing out.
Not to mention that Crosby was saying he had his meniscus "trimmed" when in reality, it was a full on repair which will probably keep him out of spring OTAs and possibly the June minicamp.