Hi. I think there must be some rules that you mentioned. I can't make it clear completely but these changes don't tend to occur on the first letter. Just to make pronunciation easy, we say for example, げたはこ　⇒げたばこ shoes box　 やきかし　⇒やきがし baked sweets So, as you used to Japanese, You'll think it just natural too. Keep on studying:)