My bet is that you're witnessing the annual flurry of activity associated with training programs for triathletes and for upcoming endurance biking events (centuries and the like). Since it takes a number of weeks to train for these, now is about the time to get started ... and bike-commuting is a good way of boosting weekly mileage.

In SJ, the streets will have a bunch of new bikers for a few weeks then it'll taper off again, if past experience is any measure.