How do airports choose which runway to use? (other than weather)?

For example, my local airport has 2 main runways: 12R/30L and 3/21. 90% of the time, 12R is used, but every once in a while, they use 3 for no apparent reason. And sometimes they use 30L instead also. 21 hasn't been used while I've lived here, but satellite shows a few (very few) touchdown marks. Remember, I know weather can be a factor, I'm wondering why they'd switch under the same weather conditions.