He didn't force throws because he was being harassed. He forced throws because he was playing from behind.
Favre takes chances, sometimes stupid ones, when he's trying to come back. Look at his situational stats. It was most identifiable in his record-breaking 2005 season. He threw 29 INTs that season, but the great majority of them (23) were tossed when his team was down. And 17 of those INTs were thrown when his team was down by 2+ scores.
The same is true this year. When the Vikings are leading, Favre's QB rating is over 100 and he's thrown 15 TDs to 1 INT. ONLY 1 INT. When they're playing from behind his QB rating is only 80 and he's thrown 3 TDs and 4 INTs. Which kind of flies in the face of all the people who brag about Favre's many 4th quarter comebacks.
The further behind his team is, the more chances Favre takes. That's the way it's been. Luckily for the Vikings, they haven't played from behind all that much this season. The Cardinals were able to accomplish what everyone should be trying to do--get an early lead and allow Favre to press.