Login to your AdWords, go to your shopping campaign & Ad Group and Click the Keywords tab. Set the time period you are looking at for a long period of time (say 12 months).
Look through the keywords and you might find some utterly bizarre searches have triggered your ads and cost you in clicks. Shopping ads are broad matched so Google has the right to pick what you search for based on what it thinks is a reasonable match. Google has always been poor at this (well, poor is probably the wrong word – it is very biased to showing more ads than seem appropriate which will earn it more money, so perhaps the right word is greedy); hence why we work very hard at using phrase and advanced broad matched terms to reign in its excesses.
Well unfortunately it seems Google’s broad match has reached a new low standard and is no better than a half-wit throwing darts at a dart board to pick keyword matches… blindfolded.
So – check the keywords and put in LARGE numbers of negative keywords to counter some of the totally idiotic matches that it will have triggered and cost you money for no possible benefit.
Unfortunately you’re only allowed 1,000 negatives per AdGroup and 10,000 per account.