xcoder - 9:43 am on Sep 21, 2012 (gmt 0)
Those are usually one page engagement for 0s (zero seconds).
BTW, the "zero seconds" thing does not necessarily mean the viewer really spent only one second. He/she may as well spent 20 minutes reading that very page but clicked back right after that.
Most tracking systems (i know of, including piwik) are able to track the "real" time spent *between page loads* only... so in other words if no other page was loaded on that very same visit then the system will log zero seconds.