Stupid umbrella was in the way?
As he opened the umbrella, the wind got under it and Witt was trying to control it.
"I--my view of the car during that length of time was blocked by the umbrella being open.
And my--the next time I saw the car after I saw it coming down on my left traveling west,
the next time I saw the car was when this activity of the car stopping, one car rushing up
on another, the motorcycle patrolman stopping, there was this screeching of tires, this
sort of thing."
I suppose Witt had already seen the President in terms of spectacle -- remember, he didn't like the Democrats and only needed a brief visual interaction with them. Then came the moment to open the umbrella, and like other stunts, it didn't go as smoothly as imagined. The umbrella first appears in the Willis05 slide (taken at Z202) and in Zapruder frame Z206, and it is quite low. The wind makes the umbrella rotate a bit back and forth. Witt would have had to control that somehow.
The umbrella is raised high in the Bronson still and film strip taken near the head shot. So maybe Witt's umbrella protest was aimed not just at the Kennedys (who he figured weren't about to apologize for Joe Kennedy's "appeasement") and more at the politicians and reporters who were in the following cars.
"Of note is material relating an umbrella sent to President Kennedy by a group of German
students, accompanied by an open letter, in protest of the reaction of the West to the
construction of the Berlin Wall and to symbolize the consequences of a policy of
appeasement like that of Neville Chamberlain"
John F. Kennedy Presidential Library and Museum