My understanding of katakana is that they are used for foreign words. I am not sure why they might use it like that:
ヒトヒラのハナビラ for Hitohira no Hanabira
The lyrics are here.
My understanding on the broader topic of styling, alphabet choice, and the like for song titles is that usually choices like this are made when they want the content to appeal to a specific crowd, or to evoke a very specific feel. If, for example, you saw the title as 一片の花びら, you might think it's a more traditional expression of the image that is evoked by the phrase. ひとひらのはなびら might give the impression the song is aimed at children due to the use of all-ひらがな. Therefore, ヒトヒラのハナビラ might at first glance give the idea that there is an alternative artistic expression of this phrase that is encapsulated in the song named after it.
Alternatively, it's to create emphasis on the specific words when read. In this case, the title is ヒトヒラのハナビラ as you mentioned, and if you come across the phrase written like this in the lyrics then you know that it's referring to the titular phrase of the song.