This is an overload failure. It's either a fluke (defective link) or tension overload. When you set up the chain next time be 100% sure that there's ALWAYS slack. You have to check for a few pedal revolutions, and make sure the lower loop sags all the way around. The sprockets are usually slightly eccentric so tension will vary, and if both are eccentric, you want to pedal enough times that the random combination of both having the high spots opposite has a chance to come up.

OR simply find the tightest position based on crank position, and make sure that there's that 1/4" each way (1/2" total) free play in the middle. That will allow enough margin for error for any eccentricity in the rear sprocket.

You might also check chainline, by standing behind the bike and sighting down the upper loop like a rifle.