The interception thrown was a result of a culmination of plays that led up to DEN baiting Kap into that interception. Throughout the 1st half of the game, Kap had shown a willingness to throw the fade/go/vertical route when he saw man to man coverage with the CB in a trailing position. Here are the four throws he made in the first half of the game when he saw this 1v1 coverage. These throws are in chronological order:
1Q 3rd & 3
Kap sees 1v1 press alignment on the backside with Crabtree.
Incomplete. Offensive PI.

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2Q 3rd & 7
CBs in press alignment. Boldin and Lloyd criss cross to get a rub/pick (man buster) as Boldin goes vertical.
Incomplete. Defensive PI.

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2Q 3rd & 8
DEN in press alignment
Kap is quickly pressured, but likes his matchup of Roby vs Boldin so he throws the vertical to Boldin (mid windup).
+18 yards
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2Q 2nd & 5
49ers: Smash
DEN: Here, DEN does not have their defenders in a press alignment.
Kap shows the willingness to throw the vertical (Corner route) when he sees that SJ has his defender in a trail position.
+25 yards
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