For, lo, they lie in wait for my soul. The emissaries of Saul were sent to David's house "to watch him, and to slay him in the morning" (1 Samuel 19:11). This seems to be the "lying in wait" intended.
Warned by his wife, Michal, Saul's daughter, David fled from his house during the night through a window, and so saved himself (1 Samuel 19:12). The mighty (or, the strong ones) are gathered against me; not for my transgression, nor for my sin, O Lord.
Not in consequence of any wrong that I have done. It is noted, as characteristic of David's early psalms, that he protests his absolute innocence in them.