What is a hive? How do they synchronize their actions?

They just carry sensor input to other agents. It means that one agent has a sensor input. It can react to it, but maybe it needs some help, so it emits some trigger to other agents, and other agents retranslate it further. The thing that they retranslate is basically the same sensor input from the first agent.

If, as an agent, you don't have that sensor input in front of you, you know that this input is carried from some other place by different agents. So this may lead to synchronized behavior because you all have the same sensor input which triggers similar actions.