I have a feeling the wife ordered the hit. She's still mad at Fisk for trying to be Mayor and he's not telling her what his plan really is. But, it could have been Fisk. Either way, I agree, this is the start of what brings him down.
It had his sort of “all or nothing”. I think Hector getting exonerated kind of broke whatever fantasy Fisk was playing out in his head that the city would just bend to his will like his empire did. He wants vigilantes off the streets, and now he’s mad the city let one right back on them (and after all that positive testimony to boot).
Which is a long way to say I think Hector’s trial pushed Fisk right back to his old ways.
Or it’s totally unrelated and plays into the “losing the grip on the underworld” subplot. Tomato tomahto