The message occurs when the computer recognises the reinforcements are too much for the system to handle. You can get a message of a similar nature when you are creating the custom battle and you pick too many units, which results in that pop-up stating your system cannot handle all that. It is not random, and and what I said is the reason for this control is in that game settings .txt file somewhere in the folder of you RTW.

There have been numerous threads on this before, but I believe the search functions on this forums was recently disabled (uses too much resources), so one cannot search for old threads through this site.