Is the fear reasonable?

On Board 28 of a matchpoint club game, EW bid briskly to 4♠ via a 2♠ opening and a raise to game.

Dealer: W #28 Vul: NS	North ♠   7 ♥   KJ94 ♦   K964 ♣   T873
West ♠   KQJ954 ♥   753 ♦   J5 ♣   65		East ♠   AT6 ♥   Q8 ♦   AT873 ♣   KQJ
	South ♠   832 ♥   AT62 ♦   Q2 ♣   A942

The opening lead was a small heart to the ace followed by a small heart back to the K and Q.  Trick 3 was a club to the king and ace.  Declarer soon drew trumps and claimed contract by virtue of six spades, one heart ruff in dummy, the ♦A and two clubs.

Obviously the defense needs to switch to a diamond before the ♣A is dislodged.

Is there a clear fault?

Yes, I think; the blame is with North.

Each defender feared that declarer could take eleven pointed suit tricks if the club ace were not cashed quickly.  While South’s fear was legitimate, I do not think the same could be said of North’s fear.  North should have played a diamond at Trick 3.

From the perspective of South, declarer might own ♦KJx or ♦Kxxx of diamonds and a diamond return would allow declarer to run the diamond suit to put to bed the ♣A (if declarer held no more than two clubs in the first situation or no more than one club in the second situation).  But from the perspective of North, the only holding of declarer that would allow declarer to take five diamond tricks was ♦QJx … and if that were declarer’s holding, then declarer would have insufficient entries to both run the diamonds and draw trumps: declarer could win a Trick 3 diamond return in hand, but must then draw trumps ending in hand to avoid suffering a ruff by South; when a second diamond from declarer is covered by North, dummy can win … but North maintains a holding that requires yet another finesse and there is no quick entry to West to take that finesse.  (Of course, if declarer held ♦QJxx of diamonds, a diamond return would be ruffed by South.)