The functions of tone change are highly complex. We can merely state that there are actually some general guidelines but definitely no hard and fast rules relating to tone change.
If the tone changes arise from morphological, syntactical and semantical factors, the rules would be more obvious while the tone changes are more obligatory. But if the tone changes arise from a phonetical factor (i.e. the effect of adjacent tones), then the rules would be less obvious while the changes are more optional. And 鞋 is one of such instances that have tone changes arising from phonetical factor. In other words, habitual usages play a greater part than phonological rules.
蛙鞋, 波鞋 and 皮鞋 [haai4] don’t have tone changes, but 拖鞋 [haai4*2] has a tone change. 車房, 廠房 and 票房 [fong4] don’t have tone changes, but 書房, 廚房 and 劏房 [fong4*2] have tone changes. The last syllables of all these bisyllabic nouns are in Tone 4 which often have a tone change. Yet there are so many exceptions that have no tone change.
So I would say that tone changes arising from phonetical ground (instead of morphological, syntactical and semantical considerations) are a result of habitual usages, and are often used optionally.
If the tone changes arise from morphological, syntactical and semantical factors, the rules would be more obvious while the tone changes are more obligatory. But if the tone changes arise from a phonetical factor (i.e. the effect of adjacent tones), then the rules would be less obvious while the changes are more optional. And 鞋 is one of such instances that have tone changes arising from phonetical factor. In other words, habitual usages play a greater part than phonological rules.
蛙鞋, 波鞋 and 皮鞋 [haai4] don’t have tone changes, but 拖鞋 [haai4*2] has a tone change. 車房, 廠房 and 票房 [fong4] don’t have tone changes, but 書房, 廚房 and 劏房 [fong4*2] have tone changes. The last syllables of all these bisyllabic nouns are in Tone 4 which often have a tone change. Yet there are so many exceptions that have no tone change.
So I would say that tone changes arising from phonetical ground (instead of morphological, syntactical and semantical considerations) are a result of habitual usages, and are often used optionally.