Not enough info, including details of the age and condition of the drivetrain, gear selected (though my money is on low), but isolated skipping isn't all that rare.

What happens is that for some reason, the RD doesn't feed the chain onto the sprocket perfectly and it climbs and rides on the tops of the tooth. That puts it at a larger diameter, so the next roller is out of phase, and 2-3 links ride the top until a roller is in phase enough to drop in as it's supposed. You now have a short section representing the loss of 1/2" (1 link) on the top, and when it comes around to 12 o'clock you get that 1/2" back in a hard pop.

If you can't imagine it, think about the last time you buttoned your shirt and missed one. The skip is like that extra bit of shirt on one side vs. the other.