By default, in OS X, when you connect an iOS device and there are new photos that haven’t been imported – if you aren’t using iCloud Photo Library – the Photos apps opens so you can import pictures. This is also the case if you mount an SD card, or any other device with photos.
Previously, the setting to turn this off was in the Image Capture app. But now, this setting is in Photos.
Connect your device to your Mac, and click it in the Photos sidebar. (If the sidebar isn’t visible, choose View > Show Sidebar.) Look up near the top of the window, and uncheck Open photos for this device.
Now, if you want to import photos from that device or SD card, you can do so by launching either Photos or Image Capture; or you can access the SD card directly in the Finder if you prefer. (Or you may use other software that can import photos.)