It came from the children's tv show Crackerjack, in which a man called Stu Francis used to shout catchphrases,:
Stu Francis also did a stand-up routine on occasion, using such catchphrases as "Ooh I Could Crush A Grape/Jump Off A Doll`s House" etc.
Stu Francis (born 1951, Bolton, Lancashire, England) is a British comedian with a camp style of delivery who achieved celebrity as lead presenter on the children's television programme Crackerjack (1979–1984), on which his catchphrase was "Ooh! I could crush a grape".
This show was really popular, so "I'm so excited I could crush a grape." became in common use, basically meaning "I'm really excited", and the rest was just for comedy.
N.B., other catchphrases from Crackerjack include:
“Ooh, I could crush a grape/rip a tissue/pummel a peach”),