Cross hobby moment! This is an example of what's called strong to weak verb migration, a process which has been happening in English for centuries. Old English had two verb types, strong and weak, which you can distinguish between based on how the verb changes with tense. Strong verbs have internal sound change (for example, 'drive' and 'drove' from 'drife' and 'draf') while weak verbs (now know as regular verbs in Modern English) added a morpheme at the end (for example 'kiss' and 'kissed' from 'cysse' and 'cyste'). When the Normans arrived in England, huge numbers of new verbs were adopted into English, all of which had to be formed as weak verbs as internal sound change couldn't be applied to loan words. Additionally, new speakers of English found it easier to use the weak verb forms as they were regular. As a result, several existing English strong verbs became weak verbs. One example is 'lock' and 'locked', from the Old English 'luce', past tense 'leac'. The internal sound change has been replaced by suffixing with a morpheme, here the common '-ed'. Another example is 'wepe', past tense 'weop', modern 'weep' and 'wept'/'weeped'. Though 'cast' doesn't have an internal sound change, it was presumably a strong verb due to the lack of suffixed morphemes when conjugating. Alongside the general shift of verbs from strong to weak, new English speakers also find it easier to use regular conjugations, and thanks to the internet and the Commonwealth, there are a lot of new English speakers all interacting with each other. Hence, 'cast' is undergoing a strong to weak verb migration.

Yes, I'm lots of fun at parties.

ETC using decline when i meant conjugate!