In The Metamorphosis by Franz Kafka, why did Gregor's dad throw apples to Gregor and what is the symbolic meaning?

In the
story, Gregor's father throws apples at Gregor because he sees Gregor as a threat. Gregor
concludes that his father has badly misinterpreted Grete's words and is acting according to his
distorted perspective of the situation. 

Basically, Gregor's father thinks
that Gregor must have tried to attack his mother. In reality, Gregor's mother fainted because
she was horrified by her son's unnatural appearance. Gregor's father does not try to uncover the
truth; instead, he corners his son and eventually attacks him by pelting him with apples from
the fruit bowl.

Although Gregor manages to evade most of the apples thrown at
him, one becomes lodged in his back. This causes Gregor great physical pain and the wound
eventually becomes infected. The infection weakens his entire body, and this leads to Gregor's
physical deterioration.

The throwing of the apples symbolizes the discarding
of all conventions of civility. Because Gregor's metamorphosis has rendered him a hideous
spectacle to human...