As others have said, it's a very snowbally game. The various characters all grow naturally stronger over the course of the game through gold (to buy items) and experience that you earn by killing minions. The problem is that killing an enemy player and destroying enemy towers grants a lot of gold and experience, so if you fuck up and die (or if you get ganged up on by the enemy team) you can end up making your opponent much stronger. Even if you live and are forced to return to base to heal, the opportunity for free farm or destroying your tower (which also makes it riskier for you to push forward) can make your opponent a lot stronger than you, which lets him kill you easier, which makes him stronger. This can also spill over to other lanes, where the opponent you made stronger starts killing your teammates and taking their towers.

There's ways to overcome this snowball--players on killstreaks are worth more gold when they die, you can gang up on a fed opponent and catch them out to nullify their stat advantage, and you can try and help other lanes to get your team stronger. The champions also have different scaling levels, and some champions get a lot of front-loaded baseline damage while others scale better with items, and a select few champions have theoretically infinite scaling (but are generally much weaker in other areas to compensate). Worst case, this means your team can play super defensive and try to wait out the advantage until they catch up and then win from there. The problem is that all this requires A) communication and the ability to quickly adapt from your teammates, B) the opposing team screws up and doesn't press their advantage, and C) your team is willing to try (which may require dragging the game out for over an hour). Needless to say, this is not always the case, and this design makes it very easy to blame another player for the loss (warranted or not).