On-list, as very basic and generally known facts: KoS tells us that the Maboder had been good Lunar allies who had refused to fight for Boldhome during the invasion.
I have no idea whether this had anything to do with the Telmori decision.
KoS p134 and thereabouts suggests that the Maboder (and other tribes) had been having trouble with the Maboder before Sartar arrived. The Jonstown confederation was formed to "settle a feud between these tribes and the Telmori" (BA p9)
Looks like "just because", "why not?" and "they're all Lunar-loving ***s anyway" would cover it adequately, but I'm sure more details would make for better story.
As a general rule for Telmori info I'd look here:
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