No reason ?
I am no expert on canine psychology but I doubt if the dog bit you for 'no reason' from its own perspective. Perhaps it was frightened of you. Perhaps it felt it was defending its territory. This is anthropomorphic language but I guess something was going on in the dog's mind that this language approximately describes.
Aristolelian chance (tuche)
Aristotle in his Physics describes how chains of events can cross - and how we call the point of intersection 'an accident', 'bad luck', 'a misfortune' and even an 'evil'. For instance there is the chain of my intentionally leaving my home, walking down the street, and stooping to tie a shoe lace. All these events are perfectly commonplace. There is another chain of events : a blind person leaves their home, walks down the street in the opposite direction to me, and falls over my crouching form as I stoop to tie my shoe lace and breaks their leg.
Here is 'an accident', 'bad luck', 'a misfortune' or even an 'evil'. There is no intentional evil in this case; I did not want to cause any harm to the blind person. But an evil occurred when the two chains of events, of which the intersection was unanticipated by either of us, crossed in this disastrous way. It happens all the time. We might call it 'hasard'.
I suggest this parallels your experience with the dog. You entered the street on your bike, rode along the street - all quite intentional and foreseeing no harm, just a chain of events. Equally the dog left its residence, trotted down a side road, turned into the street, saw you and perceived you as a threat, just another chain of events. Just as the blind person automatically fell, so the dog
automatically bit you. We can explain why the blind person fell and we can explain why the dog bit you : but both events were 'chance' in the sense that there is no general law covering the encounter of blind persons with crouching figures or the encounter of frightened or panicky dogs with cyclists.
I have varied Aristotle's language and examples to convey his general idea more clearly and connect it more closely to your question.
Aristotle, Physics, II.4-6.