In a nutshell: by 1977, Hodgson & Davies were rather well to-do from the royalties of Supertramp's songs (which they were credited as sole writers of), while the other three non-writers found themselves hurting financially despite having two huge-selling albums under their belt (and counting).
In response to their grievances, the 'Tramp's manager Dave Margerson negotiated a publishing agreement that granted Thomson/Helliwell/Siebenberg a share of
all songwriting royalties from Hodgson/Davies compositions. These royalty payments were reportedly over $100,000 a year, and continued from 1977 right up until 2018 when Hodgson, via his publishing company, ceased making payments.
In 2021 the three ex-members sued Hodgson/Davies for breach of contract.
The three settled their dispute with Rick Davies in 2023 and dropped him as a defendant.
In 2024 a California federal jury sided with Hodgson, ruling that he could terminate the agreement after a "reasonable time", meaning he was within his rights to stop payments in 2018.
Last month on 20 August, the 9th US Circuit Court of Appeals overturned that decision, holding that the contract's implicit duration extends for as long as those songs keep generating income. Essentially, Hodgson may now be required to reinstate those royalty payments to Thomson/Helliwell/Siebenberg until those songs either stop generating revenue, or enter the public domain.
That's the thumbnail sketch of what happened. If you want a deeper dive, here's a little light reading in PDF form:
https://fingfx.thomsonreuters.com/gfx/legaldocs/mypmxxxylvr/SUPERTRAMP COPYRIGHT LAWSUIT 9thcir.pdf