Here's how it happened: last week, an arbitrator ruled that the NFL must return more than $100 million to the revenue stream that goes to the players after it was found that the league had pocketed the money due to a mistake by the owners. The owners mischaracterized a ticket-sale revenue exemption that withheld $50 million in salary from the players.
So basically, the NFL tried to cheat its players out of money by keeping it from them for other uses. Those uses, according to NFL spokesman Brian McCarthy, who I'm sure is telling the truth, included stadium construction and other renovation projects.
As a result of this, the salary cap is expected to rise from $154 million to $156 million. According to the ESPN report, teams were informed in December that the cap was supposed to be anywhere between $150 million and $153.4 million. In 2015, it was $143.28 million.