My only guess would be that Sage is trying to keep the number in their correct normal balance column rather than moving it to the traditional accounting column. I.e. it's trying to keep an expense balance in the debit column regardless of whether it has a debit or credit balance.

I have seen this when it is a correction or reversal to the original entry.  I think it makes corrections easier to track. 

I have worked with Sage for many years and have humourously heard others refer to these entries as "crebits" and "dedits".

This allows the creation of a proper Statement of Cash Flows using Financial Statement Designer.  You have to turn on "track debits" in the Balance Sheet accounts in order for this to work.