For me it usually just happens randomly. Today it was because watching news which included the start of the last Ariane 5. I was mainly interested, how much "fuel" (propellant) is used and the relation between launch weight and payload weight. I quickly got distracted by rocket engines and read here and there.
Then I am currently reading a book about WW2 u boats, so I've read the Wikipedia regarding that specific type of U-boat, the torpedos, the torpedo calculation computer, the lives of several commanders, ...
Recently my kid played with clay, so I got to read a little about the process of firing clay, basically wanting to know if I can diy it.
Normally the things I want to know more about just fly in by themselves.