Looking at top100 coins on coinmarketcap and figure out a lot of incredible swings for now reason (for me as a newbie).
there is always a reason, it doesn't have to be a logical one though. it can be a simple pump, it can be some random news like some random altcoin enabling SegWit, or some illogical reasons such as releasing a new wallet!
What do you thing, how much of them is simply pumped?
all of them.
How much of such swings can be predicted based on open information?
very few. maybe 10% of them in my experience. but as we get closer to the pump itself it becomes easier to predict them based on what you see on the charts. (it becomes easier not easy).
things such as increase in the volume, the price rise itself, seeing some news on the media and things like that.
Is it any reason to focus on alts with their 20-30-40% growth over 24h?
yes, that can be a strategy. you watch the market (usually automatic) and when you see this type of rise and speculate a bigger rise you invest and enjoy the profit.
it is like catching the train mid way and jumping out before the finish line.