I think it’s already mentioned in the answer, “Now if an accident has occurred then we want the remaining $6$ accidents to occur on the same day.”
If you think carefully then $1\over7^ 7$ is the probability of 7 accidents occurring together.
However, the questioner wants to know about the probability of those accidents occurring on the same day. The only way we can ensure that they occur on the same day is by letting one accident to occur and then make the remaining accidents to take place on that day. Therefore, for the first accident, the probability will be $1\over7$, and the probability for the remaining accidents occurring on that day (this is what the questioner is essentially asking) will be $1\over7^6$.
Please note that my answer was only based on intuition. I would suggest that you first learn its quantitative approach and then think about my answer that would make more sense.
As a last note: you can easily relate this to conditional probability.
P(accidents occurring on the same day | accident has occurred) . . .