Next steps:

• Check for additional negative keywords that you may have to configure by asking NetInsight for a correlation report on paid keywords vs. actual search phrases. Sometimes, an unrelated search phrase spikes in usage and accidentally triggers your ads.
• If you are advertising on the PPC outlet's content network, ask NetInsight for referrer info and compare to previous periods to identify unwanted publishers
• Ask NetInsight for a geographic breakdown of visitors and compare to previous periods in case traffic shifted to unwanted geographies
• Investigate for signs of click fraud (e.g. frequent cookie, IP, keyword, user agent string that repeatedly clicks your ads)
• Had you recently changed your PPC landing pages? Then these changes may obviously have been adverse. Experiment with rolling your changes back.
• Had you recently changed your PPC ads or keywords significantly? Then the new set may be out of synch with your content on the landing pages therefore leading to higher bounce rates.