Hawkins' use of the word 'accident' is disingenuous, as pointed out by Paul Korshin (pages 62-65, in "Johnson's Last Days," in "Johnson After Two Hundred Years," (1986, University of Pennsylvania Press). The word "accident" means now what it meant in 1784, an event that happens by chance. Yet everything Hawkins descibes about his own actions at this time seem to have been deliberate, not an accident. Korshin points out that this ancedote was not included in the Hawkins' 1st edition, and the insertion here (as well as the defensive tone) may have been in response to negative reactions over his failure to include it in the 1st edition. Korshin has much more to say on the issue, and I encourage you to read his piece.
It's unlikely that Hawkins means that Johnson's mislaying of the personal papers is the accident Hawkins is referring to, as in a later footnote Hawkins refers to the entire sequence of events as having "so disturbed" Johnson. (Lynch)