Mahon played in 41 games last season for the Hawks, potting 2 goals and 1 assists and 133 penalty minutes.
Joe was signed on August 30th of 2012 and played the enforcer role last season. Calgary recently traded an enforcer in Defenseman Peter Kosterman to the Prince George Cougars and were in the market for someone with Joe's abilities.
Mahon is still a few weeks away from recovering from off-season shoulder surgery and may have become expendable in management's opinion due to the addition of Steven Alldridge (who will play a similar role) to the lineup this season. Joe is 19 years old and would most likely have not made the team as a 20 year old irregardless next season, so Portland getting a pick or two out of it is a coup for them.
Here's a video of a nice take down of his on Mitch Elliot: