Happen, v. i. [imp. & p. p. Happened; p. pr. & vb. n. Happening.] — Webster enters it under “happen”
Etymology: [OE. happenen, hapnen. See Hap to happen.]
1. To come by chance; to come without previous expectation; to fall out. There shall no evil happen to the just. Prov. xii. 21.
2. To take place; to occur. All these things which had happened. Luke xxiv. 14. To happen on, to meet with; to fall or light upon. "I have happened on some other accounts." Graunt. -- To happen in, to make a casual call. [Colloq.]
Shades of meaning
Fernald has no entry of its own for happening; he lists it under accident.
He lists it without comment; the entries above explain its neighbors.