Why has Boldwood's disturbance caused by Bathseba's valentine been compared with 'first floating weed to Columbus" in 'Far from the Madding Crowd'

As the remainder of this quote indicates, the meaning here is that the valentine is a small thing that hints at potentially enormous change, or  "contemptibly little suggesting possibilities of the infinitely great." 
To understand that comparison, you need to know about what Columbus and his men experienced on their voyage of discovery in 1492 -- information that Hardy assumes his audience shares.
About a week after the sailors finally lost sight of the land they had departed from, Columbus (writing in his log) wrote that they began to encounter

"large patches of weeds very green, and which appeared to have been recently washed away from the land."

This caused excitement and hope, because the men

"concluded that it must come from some nearby land. But at the same time, it caused some of them great apprehension because in some places it was so thick that it actually held back the ships."

Weeks later, they were still at sea -- longer than anyone had anticipated -- and the men were openly expressing their fears and doubts that they would ever reach land.

"Some feel that they have already arrived where men have never dared to sail and that they are not obliged to go to the end of the world."

But the sightings of floating weed continued. It was a persistent feature in their lives--a sign that a mysterious new land was near, and that they were on the verge of a huge, life-altering change.
So when Boldwood receives the valentine, it represents for him that sort of sign. We've been told in Chapter 12 that Boldwood is

"erect in attitude, and quiet in demeanour. One characteristic pre-eminently marked him — dignity."

He is the only man in the market who ignores Bathsheba, giving rise to the impression of someone "wrapt up and indifferent, and seemingly so far away from all he sees around him." This, says Liddy is probably due to a failed romance in his past.
The valentine Boldwood receives was sent as a cruel joke, but he doesn't know that. He spreads it out on his mantelpiece and studies every clue it might contain. But there is very little to go on--just a child's rhyme  ("The rose is red, The violet blue, Carnation's sweet, And so are you") on a flimsy piece of paper, and an envelope with his name written on it and a red seal bearing the words "marry me."
He obsesses over the mystery of who wrote the letter, dreams about the unknown woman of his imagination, and experiences "nervous excitability" that bothers him. He is accustomed to being dignified, standoffish, and in control. Clearly, he is intrigued about the idea that an admirer wants to marry him, and the prospect of this future represents a new world that is both exciting and a bit frightening.
https://sourcebooks.fordham.edu/source/columbus1.asp