Can be one of two reasons this happens.
(1) Lightroom has been set to automatically begin uploading when a card is inserted into your card reader.
To disable this: Lightroom preferences > General and disable the option "Show import dialog when a memory card is detected" > Click OK and restart Lightroom.
(2) If that does not work it might be set in an Operating System setting. Windows (and OS X) have Auto Play settings to Do Something when something else happens. Check your default settings to see if Lightroom was selected in the Device/Auto Play area as the default action when SD card is inserted.