I heard an interesting example of how irregular plurals can become regular when they're part of an exocentric compound idiom: sweet tooths.
In case you don't know, the idiom 'to have a sweet tooth' means that you like to eat sweet things. It's not very often used in the plural, because you're usually either saying that you (singular) have one or asking/telling someone else (singular) (if) they have one.
But of course it can be, if for instance you were talking about your two children and saying that they've both got a sweet tooth. You might say it as I just did, or you might say they've got sweet tooths. It sounds a bit weird, but it sounds even weirder to say they've got sweet teeth - the idiomatic meaning is completely lost and it becomes just an odd fact about their teeth.
The same thing happens with Walkman, the personal music player Sony makes (MP3 these days, but used for cassettes, CDs and minidiscs at various times in the past). The plural is notoriously variable. Some people will adhere to the 'man' part and say Walkmen, but most will go for Walkmans.
It's because, as I said, it's both exocentric and idiomatic. Exocentric means that it's not a type of tooth or a type of man - the 'head' of the compound doesn't match what the whole term describes. The opposite would be an endocentric compound like blackbird, which is a type of bird. And idiomatic means that it has a meaning that can't be readily deduced from its parts - so you couldn't predict the meaning if you didn't know it (because it's not a man that walks or a tooth that's sweet - it goes hand in hand with the exocentricity). These two fact together lead to a pretty opaque term, and the irregularity (which tends to erode over time anyway, as evidenced by all the irregular forms we've lost over the centuries) doesn't seem to apply any more, so we apply the default regular plural.