harry potter - Why did Scabbers bite Goyle?

When Malfoy, Crabbe and Goyle come into Harry and Ron's compartment, and Goyle tries to take a chocolate frog, Scabbers bites him:

Goyle reached toward the Chocolate Frogs next to Ron — Ron leapt forward, but before he’d so much as touched Goyle, Goyle let out a horrible yell. Scabbers the rat was hanging off his finger, sharp little teeth sunk deep into Goyle’s knuckle — Crabbe and Malfoy backed away as Goyle swung Scabbers round and round, howling, and when Scabbers finally flew off and hit the window, all three of them disappeared at once.
_{-- Harry Potter and the Philosopher's Stone, Chapter 6}

It is revealed in later books that

Scabbers is in fact Peter Pettigrew in his Animagus form.

Considering this, why would Scabbers bother going to the trouble of biting Goyle? Did years of being with the Weasleys make him somewhat loyal to them?

Answer

There's no obvious reason for Peter Pettigrew to want to bite Gregory Goyle over a Chocolate Frog. I think that Raditz_35 is right to point out that Wormtail operated out of self-interest. So he was loyal to Ron insofar as that loyalty helped to maintain his cover and thus sustain the comfortable lifestyle he enjoyed whilst waiting for news about Voldemort.

"You weren't willing to commit murder right under Albus Dumbledore's nose, for a wreck of a wizard who'd lost all his power, were you? You'd want to make sure he was the biggest bully in the playground before you went back to him, wouldn't you? Why else did you find a wizard family to take you in? Keeping an ear out for news, weren't you, Peter? Just in case your old protector regained strength, and it was safe to rejoin him..."
_{(Prisoner of Azkaban, Chapter 19, The Servant of Lord Voldemort).}

Performing these sorts of actions certainly made him a faithful rat in his own eyes.

And he scrambled around to Ron.
"Ron...haven't I been a good friend...a good pet? You won't let them kill me, Ron, will you...you're on my side, aren't you?"

_{(Prisoner of Azkaban, Chapter 19, The Servant of Lord Voldemort).}

Wormtail's actions in attacking Goyle were also respected by Ron, who honoured Wormtail as a loyal rat when he believed he was dead.

"All he did was eat and sleep, Ron, you said it yourself," said George.
"He bit Goyle for us once!" Ron said miserably. "Remember, Harry?"
"Yeah, that's true," said Harry.
"His finest hour," said Fred, unable to keep a straight face. "Let the scar on Goyle's finger stand as a lasting tribute to his memory."
_{(Prisoner of Azkaban, Chapter 13, Gryffindor versus Ravenclaw).}

Clearly, as far as Ron (and Fred) was concerned, Wormtail's actions were gallant and brave. Selective acts of loyalty such as this were worthwhile to Wormtail in exchange for staying in the Weasleys' good books when the alternative was fending for himself. He had a comfy life as a household pet, with free accommodation, food and protection. And, as Sirius says, at the Burrow he was well-placed to hear news about the potential return of Voldemort.

Besides, his attack on Goyle doesn't seem to have cost him much...

"I think he's been knocked out," Ron said to Harry. He looked closer at Scabbers. "No - I don't believe it - he's gone back to sleep."
And so he had.
_{(Philosopher's Stone, Chapter 6, The Journey from Platform Nine and Three-Quarters).}

Plot-twist speculation

There is actually one reason that comes to mind which explains Wormtail's actions, although it's not really supported by anything concrete. Gregory Goyle is the son of a Death Eater. Wormtail risked his life by switching sides and betraying the Potters. Straight after casting his lot in with Voldemort his master is vanquished and he is pursued by a murderous Sirius Black. Wormtail may have felt that he'd sacrificed rather a lot for the Dark side and not got anything back in return by means of protection or recognition. He'd had to fend for himself. Wormtail may have felt bitter towards the Death Eaters at this point. Perhaps he hadn't come into contact with any before that fateful day. Then all of a sudden the sons of three Death Eaters who walked free - Malfoy, Crabbe and Goyle - stroll into the compartment. Wormtail, filled with rage, took a small portion of his revenge by biting one of them on the finger. When he gets shaken off he feins sleep and nobody is any the wiser...