Why it will happen: The Steelers have a general lack of experience at their defensive end positions right now. Cameron Heyward has, by far, the most experience playing within this defense, and he only began starting after five games in 2013. As outstanding as Heyward was, the dropoff behind him is considerable. Cam Thomas was signed in free agency as a utility defensive lineman - more of Al Woods' replacement than Keisel's. The team drafted Stephon Tuitt in the second round of the 2014 draft, but the learning curve for him may be too steep to think he can get on the field from the first snap of the year and contribute positively.
The Steelers signed free agents and had a targeted draft of athletic players, particularly on defense. While the team started 0-4 last season, it ended on a hot streak, narrowly missing the playoffs. They are rebuilding at the same time they're competing, and could be in line for a run at a division title in the AFC North, a division in which the Steelers' three opponents all lost at least one coordinator.
Having Keisel's experience to hold down the team's right defensive end position while Tuitt matures could be the bridge the team needs to both compete now and build for the future. Throwing the rookie into the fire too early could have negative ramifications for this year's team, as well as shake his confidence as he is groomed to be this team's long-term starter. Keisel can help both the defense now as well as serve as a mentor for Tuitt. That kind of full scale contribution is worth a one-year contract.
Why it won't happen: The team drafted Tuitt and are clearly looking for him to be their starter in the future. The same thing happened last season with Jarvis Jones, and it ultimately only complicated the situation they had with Jason Worilds at the right outside linebacker position. There's a reason rookies do not commonly start in Dick LeBeau's defense - some of that is talent and skill, but some of it is the desire to start the players with the most experience.
Things can, do and will change. Tuitt's strength and technique are evidenced enough through his college tape to think he has a solid foundation already in place (moreso than Jones last year) and they could see outstanding growth in training camp. That could lead to them feeling all he needs is game reps to move him forward.
Keisel probably knows all of this as well. Perhaps he isn't interested in holding down the position for a rookie to overtake him at midseason, and would rather just retire. It's hard to blame him, if that's the case. While Keisel didn't make quarterback money over his 12-year career, perhaps he saved up enough that, while another paycheck wouldn't be a bad thing, but he doesn't want to go through the rigors of another season unless it's on his terms.
Keys: This will all boil down to how confident the team is Tuitt can play the vast majority of snaps. They won't get a good idea of this until training camp, when they have pads on, and have had more of a chance to monitor him more closely. If he shows the ability to play, the chances of Keisel coming back are reduced by the fact he may not be guaranteed a spot, certainly not a starting spot for the whole year. That may be something he won't want to deal with, and just hangs them up.
It appears the more reasonable bet Keisel will return at some point during training camp, and holds down the starting position with Tuitt playing a role similar to what Heyward and ex-Steelers DE Ziggy Hood did last season. He can work into a rotation along with Keisel and Thomas and give him a good amount of exposure with the safety net of the experienced veteran playing in the base defense. The Steelers have the cap space available and a clause in the CBA allowing Keisel to be signed at the cap hit of a second year player gives the Steelers the means and the motive to sign him.