Why does the game sometimes start before everyone has loaded?

Sometimes when loading a game, it takes longer than usual and I arrive into the game in the 2nd or 3rd minion wave.

I don't have the fastest internet/computer but I've also seen this happen on streamers videos so know its not an issue that is just mine.

The start of the game is one of the most important times for items and collecting champions and it makes it a lot harder to do well in the game if this goes wrong