ADMT causes orphaned mailbox

Let me first list my specs.

DomainA (15 GC/DC)
DomainB (25 GC/DC)

All mailboxes are on DomainB.

Using ADMT to migrate users from DomainA to DomainB.

After using ADMT, I would guesstimate about 1 out of every 7 users, a strange occurence happens. Let’s say, the user had a mailbox in DomainB, StorageGroup3. After a successful ADMT, about an hour later, the AD account shows mailbox info as DomainB, StorageGroup1. The original mailbox on StorageGroup3 is there, but disconnected. I eventually have to detach the newly created mailbox of SG1, and reconnect original mailbox on SG3.

My question is: Where do I start looking to troubleshoot, why this is happening?