The pitcher thought the batters were suspiciously good at anticipating his pitches, and then the first baseman heard voices coming from the baserunners' helmets:
That’s when [the opposing coach] went to the umpire and asked him to check two Atlantic Cape helmets. There was one out in the bottom of the third inning.
...
The umpire did, finding earpieces in the helmets of both the runner on second base, and the one on first.
...
“They had a center-field camera, which I wasn’t watching the stream, but the center-field camera, they can see in, they can see the catcher, they can see the signs,” [the opposing coach] said. “[The coach], in his dugout, had an iPad in his hands, so my assumption, and I don’t know if it’s true or not, my assumption was he was using the iPad to watch the livestream, the center-field camera, and looking into the catcher’s signs. Whether he did or not, who could prove or say, but he had an iPad in the dugout and you’re not allowed to do that.”