When does an adjective end in hu1 (乎)? To paraphrase, what do adjectives ending in 乎 have in common?
乎 usually does not form adjectives, but adverbs, that is modified verbs or adjectives. 胖 is an adjective, 胖乎乎 is an adverb, or a modified adjective, adding an enhancing element, or an injection if you will: fat to a great extent; 急乎乎: in an extreme hurry.
乎 has other uses, but with regard to your question, it is an enhancing modifier.