The premise of this strip is from the line of sentient food strips within the cannon of this comic. The joke is building on the running joke of John’s refrigerator, to suggest that the aforementioned sauerkraut has been in John’s fridge so long that it has evolved, gained sentience, and then assaulted Garfield when he went to get a snack from the fridge.
Her mother had her holding her arm like that for so long that it stuck.