A bug is causing players in Competitive Mode to quickly disconnect and reconnect to matches - but even if their team wins, the player receives a loss.
Blizzard is currently working on a fix for a nasty bug in Overwatch, which causes players to disconnect from a game. Although most players are able to reconnect just after and complete the match, they will receive a loss - which, in Competitive Mode, includes a decrease in the player's skill rating - regardless of whether or not the player's team wins. I personally experienced this bug during two separate placement matches, and ultimately gave up on Competitive Mode until a fix is available.
Game director Jeff Kaplan assured players on the Battle.net forums that Blizzard is actively working on a fix for this bug, which will "hopefully" be available this week - although specific details on the fixes, or on whether Blizzard has yet identified the cause of the bug, were not provided.
Last week, Kaplan confirmed a separate map that caused players' skill ratings to not be properly adjusted in the game's Control Maps. Kaplan called that bug a "top priority," and it was quickly addressed.