Accident or accidental ar words tied to anthropomorphic meaning and has negative connotations. Planned and intentional also have anthropomorphic connotations. None of these terms fit how Nature is natural.
From:
accident - Google Search
An accident, also known as an unintentional act, is an undesirable, incidental, and an unplanned event that could have been prevented had circumstances leading up to the accident been recognized, and acted upon, prior to its occurrence.
Nature is fundamentally deterministic and the outcome of all cause and effect events are determined by the Laws of Nature. The pattern of the outcome of cause and effect events is fractal described by Chaos Theory. The only thing that may be described as random is the timing of the outcome of individual events.
Note: I am only dealing with the nature of our macro physical existence. The Quantum Mechanics micro-world requires more of a detailed response.