"but thus" would be an equivelent of "only this" in this context
Cassius: I am glad that my weak words have struck
only this much show of fire from Brutus.
"Why" indicates disbelief that such an action (not acceptin the crown) has been taken.
Quote:Casca:Why, there was a crown offered him; and, being offered him, he put it by with the back of his hand, thus, and then the people fell a-shouting.
2.Why there is an a hyphenated with "shouting"?
"a-" does not affect how the word is interpereted. Shouting and a-shouting means the same thing. It may be a colloquialism (slang) or an affectation (habit, mannerism, quirk, showing off).
The "a-" is poetic, that is, a-shouting sounds nicer than just shouting.