Why did a SQL Server Restart roll-back seemingly committed transactions?
As per docs online:


  Returns the number of BEGIN TRANSACTION statements that have occurred
  on the current connection.


https://docs.microsoft.com/en-us/sql/t-sql/functions/trancount-transact-sql

This is a connection level variable. If there were transactions open for connections other than the one your colleague ran his query on, there could have been open transactions.