Origin of 乎 as a bound morpheme in words such as 热乎
In Classical Chinese, the word 乎has several other uses besides as an interrogative marker.  In Ch 27 of ‘Classical Chinese, A Basic Reader’ by Yuan, Tang and Geiss, it occurs with an adjective:  巍巍乎若泰山  ‘solemn and majestic like Mount Tai’ (referring to lute playing).  They describe 乎 here as ‘an enclitic particle attached to an adjective to intensify or emphasize its meaning.’ (Glossary, p. 110.)

The 漢語大字典 (p. 19) gives a similar example from the 論語:  郁郁乎文哉!  ‘What elegant words!’ There might be a link here with another use of 乎 as an exclamatory particle.

Yet another (maybe more prominent) use of 乎 is as a variant of the coverb 於, which marks locative complements and can basically be translated as an English preposition like at, in, to or from, depending on the main verb.  (See Pulleyblank, ‘Outline of Classical Chinese Grammar’, p. 53 f.)  One modern item on the list seems to work this way.  With 近, we find both 近乎 and 近於 with the meaning ‘close to, bordering on’. The FLTRP Chinese-English dictionary gives these examples: 近乎荒謬 ‘bordering on the absurd’ and 近於荒唐 (same gloss).

So, here are two possible classical sources for 乎 as an adjectival suffix.  Probably in the majority of cases it is emphatic, but with 近 it functions as the head of a complement clause. In modern Mandarin it may well have taken on additional meanings or connotations.