This could happen for two reasons:
1. previous step's completion rules are set to "auto or idle" or "idle" which means that step will be considered complete after certain amount of idle time.
2. the prompts are very loose and something in the previous step's response matched the prompt
You can verify #2 by looking at the "Structure" view for the previous step and see if there is a prompt node and what its value is.
You can verify #1 by looking at the previous step's properties and looking at the "Completion" node in the property tree.
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