A lot of it is literally just Japanese having different grammar and English translations historically being bad, and sometimes trying to account for the former by contorting the translated sentence as far as possible in English so that it begins or ends with the same word (etc.) as in Japanese because of the formatting of the text in the case of manga.

Also being overly-literal with how something is formulated in Japanese when the speaker is just speaking in a normal idiomatic way and so it would generally be better to make it idiomatic English, though that of course relates to bad translation in general.

Also also some things are just hard to translate in a way that sounds like actual English, stereotypical examples including honorifics and personal pronouns.