Lebanese, welcome to posting on FlyerTalk. As moderator, I edited the thread's title to make it a bit narrower.
Without having been a witness to the interaction, one can only speculate how this happened. So speculating, did you hand the agent the boarding pass for your original flight on United? Could it be that Airline X was planning to use that coupon's value to cover your passage. Another possibility is that Airline X knew that your were top tier on UA and wanted to try to poach your business. But it's all speculation.
A remaining question is why UA Global Services did not "rule 240" you over to another carrier (like Airline X) so that you wouldn't have had to do the legwork yourself. GS is known for offering this kind of support to displaced GS travelers. However, it could be that UA did not have a (clearing-house) relationship with Airline X. Had I been GS, I would've followed up with the GS desk to see why this wasn't offered.
Last edited by Ocn Vw 1K; May 19, 2015 at 2:58 pm
Reason: update terminology