作怪 is a separable verb (离合词) meaning 'to make trouble' or 'to cause mischief'. It often describes supernatural or metaphorical trouble (e.g., ghosts causing mischief) or things acting up (e.g., a machine malfunctioning). When splitting, you can insert words between 作 and 怪, such as 作什么怪 ('what mischief are you up to?').