El Capitan changes the behavior of how camera SD cards are handled when mounted.
With previous versions of OS X, you could use the Image Capture app to adjust what application is automatically launched when camera SD cards are mounted (by highlighting the card, and selecting from the “Connecting this camera opens” list near the bottom left). If you didn’t want any application to launch, you could select “No application”, and this would apply to all camera SD cards mounted in the future.
By default El Capitan automatically launches Photos when camera SD cards are mounted, and similarly you can use the Image Capture app and select “No application” to prevent this (or you can uncheck “Open in Photos for this device” in the Photos app). The big difference is this setting is now on a card-by-card basis, so if you are working with a lot of camera SD cards you have to make this change for each.
A lot of photographers are complaining about this change, because having Photos automatically launch all the time is a nuisance. To further complicate this, photographers frequently reformat their camera SD cards, and each time the card is reformatted this setting returns to default.