Final answer:
The behavior of William not replying to texts can be attributed to dispositional causes, specifically his personal tendency or habit of not responding to messages.
Step-by-step explanation:
Situational influences refer to external factors that affect behavior, such as the immediate environment or social context. Dispositional influences, on the other hand, relate to internal factors like personality traits and temperament. In the case of William being a terrible texter who never replies to anyone's texts, this behavior can be attributed to dispositional causes, specifically his personal tendency or habit of not responding to messages.