It wasn't the hijackers since they were working independently and had nothing to gain from it.
It could have been his business rivals (the gangsters), who mentioned they didn't like him going into there territory, but they had never done anything about it before, so why now and for what purpose.
The scene never really comes to anything so what was the point of it?
The movie does not make it clear, as it doesn't make clear who the apparent burglar was hired by. The fact that there isn't one single person responsible for all the attacks adds tension and contributes to the sense of violence and corruption, both in the city in general and in this particular business.
But I think it alludes to Peter Forente, the tennis player that Abel goes to for advice and eventually for a loan. There is talk about his father having used (and having also been the victim of) violence in the past and that they are not "nice guys". In the conversation they have about the loan he particularly mentions how they have not gone after Abel as hard as they should have for "poaching customers" - suggesting he is especially bothered by this. And the attack on the salesman (which seems to have been set up with the participation of the owner / person at the house) would be directed towards stopping / discouraging this. The fact that the salesman is put into the back of a utility truck and dumped also suggests it is something planned in advance for intimidation, not just a random act of violence ie house burglary.
Good explanation blackmailbox17...I was confused by this as well. But it's true when Abel is told they did not go after him as hard as they could...it's the gangsters (competitors) who are behind all but the hijacks (at least not the one hijacker Abel catches in the subway). Abel sees himself as honorable, always doing the right thing. But when accused of poaching, for example, he does not deny it.
At first I thought the wife was behind all the trouble. I thought she had plans to put Abel in prison and run off with the money she was skimming (or take the house, etc). She was a little confusing, because she clearly has no respect for him and sees herself as really running the show.
Did you notice the new sales staff were blonde white kids? He was giving them minimal training and sending them into other dealer's neighborhoods. The salesman got beat up by whatever dealer operated there. Abel had no respect for anyone's turf. He wants it all.
There is a deleted scene on the dvd with the younger dealer who had come from a mob family. He welcomed Abel to moving to his neighborhood in the suburbs from Yonkers. Then he tells him he saw one of Abel's trucks in his neighborhood. He tells him the driver was so blonde and white he looked like he could have been in the Hitler Youth. Abel tells him the driver is from Ohio and played football there and was picked to blend into a suburban area with other white people who had fled the cities. He would be non threatening by not being dark skinned.