Combo Bonus is a feature that automatically matches one tile every time you achieve 10 combo moves in a row.
To turn off Combo Bonus
Tap the Settings button (gear icon) at the bottom left corner of the play screen.
In the settings popup, find the Combo Bonus option.
Switch it to Off, and the Combo Bonus will no longer activate.
Combo Bonus is enabled (On) by default and can be turned off manually if preferred.