It’s literally just that the language uses compound words constructed on the spot, as opposed to compound phrases. When you say “insect death”, German grammar just dictates that if it’s written without prepositions as “insect death” and not “the death of insects”, you have to write it in one word.

The same works in Hungarian as well. “The death of insects” would be “a rovarok halálozása”, while “insect death” has to be written as “rovarhalálozás”. Every compound phrase without a preposition to clarify the relationship of the words becomes a compound word.

Actually, Hungarian is even worse, because prepositions and some other stuff also become suffixes, and are thus attached to the word. So the phrase “happening at the time when insect death is caused” can be translated word for word as “a rovarok halálának okozásának idejében történő”, but it is equally right, and more succinct to use the adjective “rovarhaláloztatáskori”.