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Eg. 厄介、欲求、直径、即興、却下、借金、引っ越す、北海道、積極、薬局

Wouldn't っ normally require the first onyomi to have a tsu ending?

Why doesn't it happen in certain words like 加湿器?
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readings gemination onyomi
In compound words what is it called when two Ks assimilate into a っ?
The name of this phenomenon is called "gemination," and it is a specific case
of a larger phenomenon that happens in many languages called "sandhi" (after
some research, there appears to be a Japanese term for this as well, known as
連{れん}音{おん}).

As for why it doesn't happen in 加湿器, I actually am not 100% sure myself, but I
would guess it's because Japanese usually resists these kinds of changes when
looking at the second pair when more than 2 things are being combined. At
least that's the case for rendaku, but maybe someone could shed some more
light on this?
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I think in general this happens to ease pronunciation, so a word isn't too
hard to pronounce then generally it doesn't have to change. I think I've heard
of variations based on modern / past or dialect.

This for example doesn't even have to a 'k' sound or a 'big つ' initially, even
words like 暖かい (あたたかい) are very commonly pronounced (あったかい), ta-ta-ka is
naturally simplified to っ-ta-ka
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> _Wouldn't っ normally require the first onyomi to have a tsu ending?_

No. This small っ is unrelated to big つ in most cases. This letter is used as
_gemination_ ([促音【そくおん】](https://en.wikipedia.org/wiki/Sokuon)) _marker_ that
you should double the _next_ consonant*1.

One source of geminate consonants is kanji that had final consonant
accommodated to Japanese pronunciation*2 e.g. 合 _kap_ + 戦 _sen_ → 合戦 _ka
**ss** en_, 鉄 _tet_ + 器 _ki_ → 鉄器 _te **kk** i_ etc. Another is reduction of
sound in some conjugation forms e.g. 勝ちて _ka **ti** te_ → 勝って _ka **t** te_,
ありたり _a **ri** tari_ → あった _a **t** ta_ etc. In the latter case, typical
examples are followed by _t_ , and that's why つ (Classical pronunciation _tu_
) was chosen as symbol of gemination.

Until WWII, both the gemination sign and syllable _tsu_ were just being
written in big つ, which made a handful of confusion such as カミツレ ("chamomile";
misreading of カミッレ (Dutch _Kamille_ )) or カムチャッカ (Kamchatka; misreading of
カムチャツカ). In post-war orthography, we can safely distinguish ordinary つ and
geminate っ.

> _Why doesn't it happen in certain words like 加湿器?_

Because, "natural" assimilation of consonants hardly happens based on today's
pronunciation. Kanji that originally had final consonant have turned into open
syllable (塔 _ta **p**_ → _to **u**_ , 鉄 _te **t**_ → _te **tsu**_ , 悪 _a
**k**_ → _a **ku**_ etc.) thus there aren't any more chances two consonants
come together. The gemination becomes more like conventional rule when you put
two or more kanji to make a word. In this case, 加湿 is already a word, so no
gemination happens between 加湿 and 器.

That said, the phenomenon is still productive between kanji that have final
`-ku` and initial `k-`*3. Famous examples include 洗濯機 (せんたっき < せんたくき), 水族館
(すいぞっかん < すいぞくかん). Theoretically kanji that end with `-tsu` could trigger the
same effect, but the problem is we don't have many words start with `ts-`
(豪雪地帯【ごうせつちたい】 _might_ be pronounced like ごうせっちたい).

Further reading: [第7期 (1964-1965)
国語審議会：発音のゆれについて（部会報告）](http://kokugo.bunka.go.jp/kokugo_nihongo/joho/kakuki/07/tosin03/)

* * *

*1: Sometimes it doesn't have a consonant following it in modern usage. In this case it represents a [glottal stop](https://en.wikipedia.org/wiki/Glottal_stop).

*2: Incidentally, [Italians did the same thing](https://damyanlissitchkov.wordpress.com/2013/03/23/how-latin-became-italian/): Latin octo ‘eight’ > otto, Latin maximus ‘greatest’ > massimo etc.

*3: The reason is usually attributed to so-called ["vowel devoicing"](https://japanese.stackexchange.com/q/1095/7810) that virtually eliminate the vowel under certain conditions, cf. すこし > [skosh](https://en.wiktionary.org/wiki/skosh).
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