Not sure if this only applies to Windows 10 users, as for I changed just last week and this bug/glitch appeared. While I'm paused, to alt+tab out to check something else, in osu! registers it as if I'm pressing tab to remove the leader board, even though that's not my intention. As shown below.
osu! version: 20160409cuttingedge
osu! version: 20160409cuttingedge