Why do you think Winnie is reminded of her grandfather’s funeral when she is talking to the stranger in Tuck Everlasting?

The stranger seems formal and acts suspicious.

Winnie is suspicious of the man in the yellow suit because he is dressed formally and acts formally.  You would not expect either of these things from the average passer-by on the street.  She is reminded of a funeral when she sees him because of how is dressed and how he behaves.

His tall body moved continuously; a foot tapped, a shoulder twitched.  And it moved in angles, rather jerkily. But at the same time he had a kind of grace, like a well-handled marionette. Indeed, he seemed almost to hang suspended there in the twilight. (Ch. 4) 

At first, the man has Winnie almost entranced.  Then the man’s unusual behavior makes Winnie think of “the stiff black ribbons they had hung on the door” during her grandfather's funeral.  She looks at the man more carefully.  She asks if he wants to see her father, and when her grandmother sees her talking to the man she is suspicious of him too. 

The man on the other side of the fence bowed slightly. "Good evening, madam," he said. "How delightful to see you looking so fit."

"And why shouldn't I be fit?" she retorted, peering at him through the fading light. His yellow suit seemed to surprise her, and she squinted suspiciously. (Ch. 4) 

Winnie’s grandmother is ready to run the man off until they hear music coming from the woods.  She gets so excited that she forgets her suspicion, and the man in the yellow suit is interested to know that she has heard the music before.  She says it is elves, but Winnie thinks that it is a music box. 

The music box is important because it is Mae Tuck’s box, and it is a connection to her past.  It is also, ironically, how the man in the yellow hat finds her.  He wants desperately to find the Tucks so that he can make money off of the immortal spring.