Wikipaedia says: "..it was also common to include a decorated bundle of broom at weddings."

So:- another wild guess - perhaps it's about tying a bunch of broom together with a length of bent, or grass, in anticipation of a wedding?

Or, as a bend is a knot (who knows, maybe originally tied with a length of grass or reed); and Free Dictionary gives Bend - [Middle English, from Old English bend, band, and from Old French bende, bande, band (of Germanic origin;], maybe simply tying the broom with a knot (or band - which might explain sophoclese's "lay the ban")