Chooses sides

Hi!

I was wondering if you could have a look for me in this sentence in red color below. What does "chooses side" mean in this context?

Does D.B. Cooper mean "Everybody chooses to stand on each of the sides, that is to say, not to take part in the fight." Or "Everybody chooses the side (the fight side) which they are on ?? What is your opinion? Thanks for your help.

(Prison Break)
Michael: You ever think about Boston?

D.B. Cooper: Sure.

M: Think you'll ever see it again?

D: I'm a 60-year-old man with 60 years left on my ticket. What do you think?

M: I'm thinking about going.

D: Well, there's goin' and there's goin'. Which one you mean?

M: The one you think I mean.

D: Three days inside, and he's already thinking about turning rabbit. It'll pass. It always does. There's bigger things to worry about at the moment. I've been in here long enough to know it when I see it. The calm before the storm. Whites and blacks are going at each other real soon here. Everybody chooses sides, and a lot of guys bleed.

M: There a reason?

D: Same reason you don't put cats and dogs in the same cage. They don't get along.