I was reading the wikipedia of the event and it makes it seem like it was done in a deliberate and calculated manner. (RFK's advice, media's "determined effort" to refer to him as President Johnson) Was there actually worry that the transition would be contested? Wouldn't there have been enough precedent set by previous assassinations/deaths in office?
The page on Johnson states that they suspected a conspiracy and Johnson himself wanted to be sure they were projecting that the US government was stable. TR was rushed down a mountain on stagecoaches and given the oath in a supporter's house without the benefit of a bible, and Jackson Johnson (wtf, brain?) was sworn in not four hours after Lincoln died. Is this really that abnormal?
He didn't actually have to take the oath (cf, the 20th Amendment), but as a consummate politician knew that this photo would demonstrate the continuity of government, put to rest any fears that the US government had been compromised, and that the swearing-in formality would show the world that he was, indeed, the President.
The urgency of swearing in a new President was nothing new.
When Calvin Coolidge heard of Warren Harding's death, it was the middle of the night. He was away from Washington, at his family farm in Vermont. There was no telephone or electricity in the house. He was sworn in by his father, a notary public, at 2:47 a.m.
This page from the National Archives has the following detail (emphasis added):
Across the road at the general store, Coolidge discussed the ceremony by telephone with Secretary of State Charles Evans Hughes (also next in line to the presidency). Hughes urged the Vice President to take the oath of office immediately.