May be the clicks were flagged as "invalid" clicks by Google. Not all clicks can definitely make money
Clicks were very likely invalid. Could be someone was wanting to perform malicious acts on your site, or a user that just clicked an ad a bunch. Either way, you should report it to adsense here.
Most definitely invalid clicks. Google is strict when it comes to that but it's for the better as it saves you the trouble of dealing with click fraud.