Why is there a boom in temporary hiring?

Trevor Frankel offers his thoughts:

The part that most clearly indicates policy is the problem is the temp hiring. If businesses are hiring temps off the bottom of a recession, it means that they're seeing demand pick up but they're too uncertain to actually hire someone full time. This is usually followed by permanent hiring, but in this recovery, it has not been. The only obvious culprits here are a) higher required wages and healthcare costs (minimum wage, housing interventions and Obamacare are prime candidates here) and b) general lack of confidence in the economy and policy (the political climate in general is the prime candidate here).

Here is some background:

Temp hiring has been off the charts – temp jobs are up 20% year over year, while permanent private sector jobs are down 1%. (source: http://www.ritholtz.com/blog/2010/08/follow-up-on-temp-services-hiring/)

This is not an argument which I have been pushing, but the boom in temporary hiring does lend some support to it.