Does anyone know if the title is realted to the happenings of this poem?
Yes, it is! Hap means chance - the etymology is thought to be from the Icelandic (meaning fit) and we get happen, haphazard, and probably happenstance from it. There is a second meaning - to cover up, or a covering, which Hardy might also have had in mind, but the main meaning is chance, and of course the poem is all about how much easier the narrator would find it to bear trouble, if he thought it had a purpose, even a malevolent one.
Ah, thanks for that. I couldn't work it out, though i thought it must have something to do with the poem.