It is so often that you should lead a trump. Many consider a trump to be the obvious alternative against slam, so did west at this hand
Let’s try out Your skills on lead, what is Your Choice looking at this type of bidding and Cards?
West listened to the bidding where south asked and north told about a weak-2 in hearts and short diamonds. Based on this it was likely that south was looking for a diamond ruff or more.
He led 7 of spades, the Whole hand looked like this
The king lost to the ace. Ace of diamonds a diamond ruffed in dummy then a heart back to the ace was followed by the jack of spades. West took the queen but could not deny south entering his hand to pull trump and claim the contract.
West was close to find the killing lead, do you see how the contract could have been defeated?
How about leading the queen of spades?
On this lead south have no play. If he holds back the ace west just continue a trump and a diamond has to be lost. If south sin the ace, then try ace of diamond a diamond ruff. A club ruff then jack of spades. East will enter on the king then giving west a ruff when play a diamond.