In state transition diagrams, what typically causes a transition from one state to another?

In state transition diagrams, transitions between states are primarily caused by events or input triggers. These events can be anything that occurs externally or internally in a system, prompting a shift from one state to another. For example, a user clicking a button, a timer expiring, or a specific input being received can all serve as triggers that instigate a transition.

By focusing on events or input triggers as the primary catalyst for transitions, state transition diagrams effectively model the system's behavior in response to different stimuli. This approach helps in visualizing how the system's state changes in reaction to various inputs, thereby capturing the dynamic nature of state changes in response to real-world interactions or conditions.

The other options, while relevant concepts in system design, do not serve as the fundamental cause of transitions in state transition diagrams. Actions specified by an actor may affect the system's behavior but are distinct from the triggering events. Conditions that must be true can describe valid states or the preconditions for a transition but do not inherently cause it. Lastly, the end of a processing cycle might relate to states and their transitions, but it does not cover the broader scope of input-driven changes that state transition diagrams aim to illustrate.