If I had to explain it, based on observation rather than having seen any actual data, I'd be tempted to say this:
Economy sinks --> many new advertisers decide to give AdWords a try - either as business owners looking for more traffic to their own sites, or as affilates hoping to be paid for delivering targeted traffic to sites that are not their own.
suggest you provide your manual reviewers with a past approval rating so advertisers that have a very high compliance and appropriate targeting history are somehow streamlined or given provisional pre-approval.
work with mechanicalturk to up your manpower and lower your costs for these reviews. gawd, i hope hyper smart stanford grads aren't reviewing banners - they need to design a system to have it done. and that image recognition company you guys bought a few years back, get that into the mechturk process so it can both learn and speed things.