Brennan joined the Suns on Thursday, the second Lions player to be signed by the new franchise after Michael Rischitelli.
Petroro said Brennan's decision to leave was centred around his need for a new challenge after a few injury-affected years, as well as the financial aspect.
"It stems to 12 months ago. When you bring five uncontracted players in, and in Brisbane's case they only really had one or two fall out, it puts pressure on the salary cap," Petroro told afl.com.au's Trade Week Radio.
"By doing that, we've seen Michael fall out and now Jared.
"It's not a reflection of Jared's disinterest in the Lions; they'll still form an important part of his life but I think it was more from a salary cap [point of view], they were really tight.
"It was time for Jared to really launch into the next phase of his career and for him, he probably needed a fresh start."
Petroro said Carlton defender Andrew Walker was another player who was looking for new opportunities, with a desire to play in the midfield behind his decision to push for a trade.
He said Port Adelaide and the Western Bulldogs - in that order - were interested in his services.
"We saw this year he probably didn't get some of the opportunities he probably desired and I think with him, it might be time for a fresh start," he said.
"The poor guy has had three shoulder reconstructions and a broken collarbone and maybe it's time to look at an opportunity elsewhere.
"It may be dependent on what happens with (Brisbane Lion) Justin Sherman. It is possible [the Bulldogs] could go for both; it may be dependent on how many players go out of their list."
Walker has met with Bulldogs coach Rodney Eade and been impressed with the club's outlook and direction.
His teammates Shaun Grigg and Sam Jacobs are also looking to leave the Blues, with Richmond the front runner for the former and Adelaide the latter.
"How [Grigg] gets done will play out over the next couple of days," Petroro said.
"Whether that's a draft pick or a player for player swap ... my read on it all is he played the last nine or 10 games this season but didn't get opportunities early in the season.
"Basically he's at a stage in his career where four years in he's come through a draft with other midfielders like Travis Boak and Joel Selwood, who he played with at Vic Country, who have played 80-odd games.
"Shaun wants to go somewhere where he's going to play every week and he's looking for an opportunity hopefully at Richmond."
Petroro also said the fact that three players wanted to leave Carlton was not an indication of a problem at the club and more a reflection of the Blues' depth.
