(version 2.15 > )
This is a new feature with the 2.15 version.
This prevents accidental dismissing or snoozing.
Slide is only asked when the phone was standby when reminder fired , and phone was waken to display alert.
In settings you can disable this , under Calendar snooze -> menu -> settings -> ui settings -> use slidelick when waking.
(only possible on paid version , and when wake device is enabled)