In commenting upon the proposed Brotherhood League, the Toledo Blade the other day gave Henry V. Lucas' real reason for forming the Union Association. In 1883 Mr. Lucas and a party of friends visited the St. Louis base ball grounds. The afternoon was stormy and the club management refused to issue rain checks. Mr. Lucas and his companions were highly indignant and then and there resolved to form an organization of their own, which they did, and it cost him $75,000.
-Philadelphia Inquirer, October 30, 1889
I'm not sure I'm buying this but it's possible. William Spink was bringing up the issue of rain checks in 1876 and by July of that year, the Brown Stockings had begun issuing them if a game was called because of rain before five innings had been played. To what extent that policy carried over to Von der Ahe's Browns in the early 1880s is unknown but, according to Peter Morris in A Game of Inches, there was at least one instance in 1883 when a ballpark crowd wanted rain checks and Von der Ahe refused to issue them, although the specific circumstances are unclear.
So when I say that this story is possible, I really mean that the rain check policy was not completely developed and it's possible that Lucas went to a ballgame in 1883, thought he deserved a rain check and didn't get one. However, human action is complicated and I think it's unrealistic that this one incident would motivate Lucas to undertake a project as large and risky as the formation of a new major league. It may very well have played a part in his decision-making but I find it difficult to believe that it was the most important factor. Lucas' love for baseball, his ego, his ability to financially undertake the project and the roll that his family had played in St. Louis baseball all played a part in his decision to form the Union Association. He also specifically mentioned the unfairness of the reserve rule as being a motivating factor, although that may have been an attempt to justify the raiding of other clubs. Regardless, it's unrealistic to describe something as complicated as the UA venture as being motivated by Von der Ahe's refusal to issue a rain check.