After a few days of weather like we saw this week, the system has crews, planes, and passengers out of place. In many cases, swaps occur to get one or more of those things back into place as quickly as possible. In some situations, there are multiple flights/AC involved in a swap. I have no insight into why your aircraft was swapped, but WN runs very few connections over DTW. If you have to delay a flight, it's best to pick one with the lowest downline impact. I could fill paragraphs on the complexity of airplane and crew routing, but to address your main concern: the swap may very well have been an effect of weather from the previous day. (Also, crews and employees hate swaps just as much as passengers, but it's part of the package.)

SM Disclaimer: The views above are my own, and not necessarily those of Southwest Airlines.