Could be a "micro" bike boom.
The first warm days of the year always draw out a few people on weekends, the ones who have been patiently waiting months (or years) for "the perfect 72 degree day with no wind, chirping birds, puffy clouds, and a sunny blue sky" to get on their dusty unused bike for an idyllic 4 block ride on their 35 pound clunker with a heavily-padded seat. Once they get that out of their systems, they're good for the rest of the calendar year, and you won't see them again until next spring.