Event driven is the normal way in which simulators for wireless sensor networks work. In this kinf of simulations, a series of events define the operation of the whole network. For example, nodes are scheduled to gather data from their sensors every five minutes, and send them towards the base station. In this example we have at least three events: the first one is a timer that defines the periodicity with which sensors have to take data, the second one is that, once sensors have gather the data, nodes have to send the information towards the base station by using the wireless channel, and the last would be when the base station receives the data from the node, it will may send an ack message.

If you are looking for some event-driven simulators, you can have a look at:

http://www.cse.wustl.edu/~jain/cse567-11/ftp/sensor/index.html