I have, also, experienced this and do not know why it happens. I assume the OS simply allocates to the first available letter.
Did you know, however, that you can change the letter back to the original, if you wish?
Surface Pro (2017) vs Surface Pro 4
20 groundbreaking 3D animation technologies coming to Siggraph 2017
iPad Pro 12.9 vs Surface Pro 5
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.