1.
If someone sneaks up on you, they try and approach you without being seen or heard, perhaps to surprise you or do you harm.
I managed to sneak up on him when you knocked on the door.
= creep up
PHRASAL VERB : V P P n
2.
If something sneaks up on you, it happens or occurs when you are not expecting it.
Sometimes our expectations sneak up on us unawares.
PHRASAL VERB : V P P n