What causes it.
How do I keep it from happening again?
Almost new bike, Sora triple. Going up a steep hill and shifting down to the small chainring. No major damage
A few factors cause chain suck - bent tooth on chainring, drive train misalignment, bad/stiff link(s) on chain, frame design.... What brand/model frame and chain do you have? Some say that shifting under heavy load (slow cadence) contributes to chainsuck too - so make sure you get the cranks spinning before you shift down.
In the past, on a mountain bike, I noticed chainsuck was more likely to occur with a muddy/gritty chain. However, chainsuck went away when I started cleaning and lubing the chain every 3rd ride or so. I use a "dry" lube, too. So I have some experiece with a dirty chain syndrome.
I read (in www.mtbr.com) that Gary Fisher (especially their 2004 Cake models) were very prone to chainsuck. I contacted Fisher and they said it was due to excessive play in the rear suspension pivot, implying alignment. (Fisher did repair all their remaining stock and I bought one.)