The popcorn effect is when you send a command to a group of lights and the lag between when different ones actually change is visible in the room, so someone in the room notices the lights coming on individually. There is video in the following thread:
There is also some detailed technical discussion of Hue polling and delays including comments by smartthings staff in the following:
There are several reasons why this can happen with hue bridge in SmartThings. Including an overworked repeater somewhere in the mix.
Often the problem is solved by using a Hue scene rather than trying to create a group of bulbs with third-party software.
In this particular case, the issue is that people had smartthings automations which had been working just fine, in some cases for months, and then all on its own the popcorn effect began. A number of people were affected at about the same time but there were no similar reports in general Hue forums. More details in the first thread above.