Current Guava based implementation queues events, and that implies changes to the execution flow.
Axon Event Bus implementations simply dispatches events as soon as are posted.
That way, logic when dispatching events is always the same (immediate dispatching when posted), without depending on an Event being dispatched when no other Event is dispatching (so its Event Handlers would be invoked immediately) or when other Event is currently being dispatched (in which case Guava would queue it instead of immediately dispatching).
As now implemented, the implementation can be specified in isis.properties:
isis.services.eventbus.implementation=guava # the default
isis.services.eventbus.implementation=com.mycompany.myapp.MyEventBusImplementation # implementing EventBusImplementation as defined in the Isis applib.