At first, the move seems a bit surprising, but a closer examination reveals that it actually could make sense on a couple of different levels.
1) The Rockies might want to push his clock back. Remember, Kahnle was selected in the Rule 5 Draft before the 2014 season, meaning he had the 25-man roster for the duration of the year. This means he hasn't been sent to the minors since breaking camp with the Rockies last spring, and as a result, he has exactly one year of service time. If the Rockies keep him in the minors until at least April 17th, they push his free agent clock back a year because he could only reach a maximum of 171 days in the majors in 2015, which is not enough to get him to two year of service time.
2) It could be an excellent sign for the strength of the bullpen. As hard as it is to believe after last season's train wreck, the Rockies just might be coming out of spring with an extremely strong pen. So strong in fact that the Rockies feel they could have seven, or even eight guys better than Kahnle in the bullpen. Let's take a look at the relief arms that haven't been optioned yet:
From the left side:
Christian Friedrich (who is out of options and can't be sent down to the minors without being exposed to other teams)
From the right side:
Rafael Betancourt (could be added back on the 40-man roster after looking excellent this spring)
That's a pretty solid group, especially with how they've looked as a unit this spring.
We now know have a very good idea of what the bullpen's going to look like. If the reports of a potential eight man pen are true and Betancourt is a part of that, expect either Brown and Diaz to be demoted. If the Rockies go with the more traditional seven man pen, there's a good chance both could be sent down.
3) It's also possible the Rockies didn't like Kahnle's performance this spring. He's only appeared in five games, but his numbers are solid (2.08 ERA and six K's in 4.1 innings pitched), albeit in ridiculously small sample size. If the Rockies don't like something or are trying to make adjustments, (and they might be because Kahnle hasn't seen much action this spring) that project is going on behind the scenes.
Last year, Kahnle spent most of the season as one of the most reliable members of the bullpen, taking a 2.61 ERA into the first week of August. However, after that point he appeared to run out of gas as he experienced the full grind of a major league season for the first time. In his last eleven outings, opponents hit him to the tune of an .874 OPS and ballooned his ERA to a less impressive 4.19.
After throwing the ball the way he did for most of last year though, don't be surprised if Kahnle finds himself back in Denver before too long.