Large increase in search traffic bounce rate post migration

We migrated our site to about 10 days ago. Everything went relatively smoothly. Migration was from a combintation of Mambo and vbulletin to Xenforo with Wordpress.
The good news is that traffic is up, it is up a lot. A good day for us was around 6,500 visitors and yesterday was 8,500 visits (Hooray!)
We are now seeing a huge increase in bounce rates from search traffic. Referral, direct and social is around the same or higher, but search, which is a large component is bouncing at 75% up from 35%

We conducted the process in google webmaster tools for migrating a domain.
We have conducted searches and they seem to resolve OK.

The site has about 500,000 URLs, and google is still indexing it madly, it did 100,000 URLs as off a couple of days ago, according to webmaster tools.

Does anyone have any ideas about the bounce rate?