They are both predators. TYPICAL predator behavior is to attack prey, and only fight other predators if they are (a) preventing you from getting scarce prey, or (b) weak enough to be a "cheap" source of meat.
So why would pteracuda and sharktopus attack each other instead of hunt for separate prey (which seems to be quite plentiful for both)?
two predators attacking each other makes sense if you assume they are territorial species
some animals not necessarily predators will lay claim to a territory and viciously defend it (cape buffalo are a good example)
A wolf pack entering the territory of another pack could also be used as an example although in this case the likely outcome would be the alphas of each pack would fight and the packs would join together under the winner
Well, I figured out an answer for Sharktopus side: it was controlled via neurotransmitters.
From the script:
This is an earlier prototype that injects the neurotransmitters right into the creature's brain.
Now we don't need remote control. It's got a one track mind to seek and destroy Pteracuda.
ScienceorNaturethat addressed this very subject.