Random speed boosts

Drafting, also known as slipstreaming, is a technique that allows for a short speed boost. If a player trails behind another racer for a certain period of time, the player's vehicle receives a temporary speed boost. If the position is held for about 3-5 seconds, the player will be surrounded in a blue wind with a modest speed increase. The boost lasts for about 3 seconds, or until the player hits a wall, obstacle, racer, or item. It can last as long as a Mushroom boost.

Screenshot from Mario Kart 8 of drafting effect:

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If this is online, it is more than likely lag through the lack of good internet/signal from one of the players. If not, it may be a side effect on one of the tracks. Or it could be drafting..