First blood gives a 60% winrate and a 40% loss rate. So just being the first team to have a death means you lose 10% of your odds to win. Okay technically a 20% loss in _odds,_ but a 10% loss in probability to win.
Taking the first dragon gives you a 68% winrate. So if you get pushed in early botlane, tough luck, that dragon loss means a game loss 18% more often.
Taking Rift Herald gives a similar 70% winrate, mostly due to the fact that the team which is already winning has a much easier time taking Rift Herald in the first place. Not so much a cause of snowballing as a symptom.
First Tower gives a **72% winrate**. So once a tower is down, that lane gets behind and can not catch up on farm because they now have to ward twice as much to keep on top of the minion wave safely. Meanwhile a first tower means that advantage starts spreading to other lanes, getting them ahead while the behind team tries to catch up.
Meaning if you die once in bot lane, your tower will fall, and you lose 22% to your odds of winning. Playing blackjack with no basic strategy or card counting gives you better odds of winning (42% vs 38%).
So yes, it turns out the games are in fact ridiculously snowbally. These factors have always contributed to a rise in winrates, but in the past, first blood was something like 54% to win, and first tower 60%.
There is no such thing as a comeback in 2018.
Oh and for fun, turns out warding doesn't matter anymore* because a single mistake in laning makes a bigger difference than the impact an unwarded river does now.