I just checked in my rafters, and the original keel from my '46 Otca is still almost perfectly straight. It's been in my rafters for 3 or 4 years, and was in the previous owner's rafters (outdoors, under the roof of the carport; Chicago area, so cold, dry winters & hot, humid summers, with dramatic changes even within seasons) for more than two decades. Different pieces of wood react differently, and you can't always tell by looking at the grain whether a piece will warp or not.
While your keel was secured to the canoe, the screws & other structures kept the warp in check. Once released, all the stresses in the wood started to push, pull, twist & turn...
Changes in humidity helped it all proceed. The seal created by paint & bedding compound can be compromised. The keel has been through a lot of years when it was submerged and dried a lot of times. It probably didn't take much of an ambient humidity change in your rafters to initiate the warping process.
I'd be curious to know if anyone out there thinks it might be worth steaming the keel, and then re-attaching it to the canoe, allowing it to dry in position before painting it? Or steaming it, then setting it up in a press, to try to get the bends out? It shouldn't be all that different than steaming gunnels.