why were the men. who followed mr. hyde to cash the cheque at coutt's bank. surprised

The men were surprised that the signature was genuine and not a forgery. Dr. Jekyll was a man of great reputation, whereas, the man to whom he wrote the check (Hyde) was a man that no one else would have anything to do with.

"I see you feel as I do," said Mr. Enfield. "Yes, it's a bad story. For my man was a fellow that nobody could have to do with, a really damnable man; and the person that drew the cheque is the very pink of the proprieties, celebrated too, and (what makes it worse) one of your fellows who do what they call good. Black-mail, I suppose; an honest man paying through the nose for some of the capers of his youth. Black-Mail House is what I call that place with the door, in consequence. Though even that, you know, is far from explaining all," he added, and with the words fell into a vein of musing.