Good question!

So here’s the relevant passage:

“He is writing each a binder. If their ships are lost in a storm or taken by pirates, he promises to pay them for the value of the vessel and all its contents.”

“Is it some kind of wager?”

“Of a sort. A wager every captain hopes to lose.”

“Yes, but if they win …" 

“… they lose their ships, oftimes their very lives. The seas are dangerous, and never more so than in autumn. No doubt many a captain sinking in a storm has taken some small solace in his binder back in Braavos, knowing that his widow and children will not want.” A sad smile touched his lips. “It is one thing to write such a binder, though, and another to make good on it." 

Emphasis mine there. The old man in question has been refusing to honor maritime insurance contracts – which the Braavosi call “binders” – and probably a widow or widows who’ve been screwed over “came to the House of Black and White and prayed for the god to take him.”