Since both ACF and ARI are controlled by Carl Icahn It seems at least at a corporate level there isn't any bad blood.
The ARI website shines a little light on the connection, It's less "spin off a competitor" and more that the two companies are divisions of the same while legally being separate companies, as for the why side of the question I would only be speculating...
1994
American Railcar Industries, Inc. was formed from the acquisition of railcar component manufacturing and railcar maintenance assets from ACF Industries. At that time, the manufacturing base was focused on supplying parts and components to rail and non-rail related industries. The Kennett Manufacturing plant was also launched this year as a fabrication and welded railcar sub-assembly supplier. The railcar maintenance assets have been utilized by ARI as railcar services facilities.
http://www.americanrailcar.com/Home/About#