Leaks almost always come from either team executives or agents of the players, both as tactics for advantageous negotiating.
If you found out a player was offered a contract with weird language forfeiting guarantees and he balked at it, that’s 100% the agent trying to save face for the player by giving a reason he “had” to leave his home team. When you find out about a player with a no-trade clause “leaning” toward one team or the other, that’s also definitely from the agent, trying to entice other teams to up the ante.
If you hear a team is about to cut a player, that’s most likely from the team trying to garner interest before he hits the open market. Especially if it’s well in advance. Teams aren’t going to contact agents 2-3 weeks ahead of It’s not always a bad thing to announce it beforehand, and the benefit is teams may feel they’d lose a free for all for the player, so what’s a measly 6th rounder to guarantee you get him?
Some teams request agents stay quiet about negotiations and trades, some teams want it out there. Dallas is definitely the latter. Doesn’t mean it’s inherently wrong, it’s just one of many strategies.