Here’s an example of how irregularities arise (from John McWhorter’s Defining Creole, p 61: “A less familiar example is in Lahu, where an erstwhile causative prefix s- eroded long ago, leaving devoicing of initial consonants and tonal disturbances on about a dozen verbs, including ’do’ =â€˜drink’ and ’to’ =â€˜give to drink’. Meanwhile, lexical verbs such as te â€˜do,make’ have been recruited into a new causative construction, which may well develop into affixes over time”. That’s similar to the causative in English made via umlauting, so: fall > fell = cause to fall. Now unproductive, that method has been replaced by lexical items â€˜make’ (make it fall), â€˜have’ (have it cut down), etc.
There’s a good example from Maori I posted to the blog earlier I’ll try to find it.