By default, services are invoked in asynchronous order. Let's say there is service A and service B. Let's say both are invoked in order on button click:
There is no guarantee that service A will finish before service B is invoked. Basically, service A will be invoked and the execution will move to service B. If you need to ensure that service B is only invoke after service A is finished, then use service A success event to invoke service B.