Here's a springboard article, if you want to do your own research.

https://www.nzz.ch/english/how-the-myth-of-stockholm-syndrome-came-from-a-media-driven-hostage-spectacle-ld.1752897

The woman, based on whom the term was coined (the psychiatrist never even talked to her) wrote an autobiography "I became Stockholm Syndrome".

There's also the works of Allan Wade, a Canadian psychologist, who has talked to the victims throughout his career.

Basically when you're at the whims of an armed lunatic, you might cozy up to them in order to appease them. The victims were also really afraid of the police coming in and shooting them. Which is pretty justified, considering the police couldn't even identify the perpetrator before conceding on his demands and bringing in his prison buddy.

The guy with a gun, whom they've been talking to for days and has not hurt them in the slightest looked much less dangerous than the impending doom of the police barging in and shooting the wrong person.