That's a known problem discussed over at /t/ideasforsnapzu I believe. The short answer is, the PHP framework used doesn't currently support staying logged in at multiple devices. It'll take modifications for that to work.
The long answer is suspected to be: a unique hash of a user is stored in the database, based on their IP address and other information. When you log in on another device, it overwrites this hash with that new login, effectively logging out the previous user.
That's a known problem discussed over at /t/ideasforsnapzu I believe. The short answer is, the PHP framework used doesn't currently support staying logged in at multiple devices. It'll take modifications for that to work.
The long answer is suspected to be: a unique hash of a user is stored in the database, based on their IP address and other information. When you log in on another device, it overwrites this hash with that new login, effectively logging out the previous user.
-- Edited for inaccuracies
Ah thanks for the explanation, I was suspicious something like that was going on, but wasn't sure.