spring upon (?.)

• ambush
• astonish
• bushwhack
• catch at
• catch off-guard
• catch unawares
• come from behind
• come upon unexpectedly
• do the unexpected
• drop in on
• fasten upon
• jump
• jump at
• pounce upon
• pull up short
• scramble for
• seize on
• snap at
• snatch at
• spring a surprise
• surprise
• swoop down upon
• take by surprise
• take short
• take unawares