Head Cannon #1 – Hyde mauls Sir Danvers because he was carrying a letter that had Utterson's name on it. He also used the cane the Utterson gave to Jekyll as a gift. I don't know if he knew that but maybe he did. Sir Danvers either asked him:

1 How to get to the post office

Or

2 Where Utterson house was to deliver it personally

I think it was 2. Why would anyone want to deliver a letter at night to the post office?

Head Cannon #2- Jekyll's transformation in front of Lanyon proved Lanyon wrong and by showing it to him he was saying "Ha you're wrong and I'm right. What do you say to me now?" Lanyon should have fainted because he literally couldn't handle the truth.

Head Cannon #3- Lanyon is loud and is dramatic on his death bed and so stubborn. I don't know what Jekyll sees in him for a friend. Maybe for his medical advice. Maybe I'm just biased by how he's described.

Maybe he should have gone to Uttersons instead of Lanyon's. Utterson could have maybe taken the shock better than Lanyon did.

Uttersons fear of his friend is understandable and maybe he had a feeling of helplessness. If Jekyll couldn't tell him what was wrong what could he do?

Utterson should have gone where Jekyll was and say "I'm going to sit here until you tell me the truth." Jekyll would eventually crack under the pressure.