Eighteen people were too close to a tower which collapsed and killed them. (Luke 13:4)
Warren Wiersbe writes in his book, Why Us: Why Bad Things Happen to God's People:
“At least in the world we know, it takes trials to make something beautiful and useful out of the raw materials of life. The student’s struggle with truth develops his intelligence; the athlete’s struggle with his records and his opponents helps to develop his muscles and coordination; the musician’s struggle with more difficult pieces develops his playing skill; and the soul’s struggle with the trials of life helps to build character.”
Another pastor puts the explanation like this:
The incidents are sad, but there are answers. Too often people are quick to think of God as causing such evil because He is not loving or not understanding. Yet that is not the case. Rather than asking "Why?" maybe the better questions to ask are "What if that had been me? What if I no longer had any more chances to respond to God's offer of salvation? When will I realize my days are short and my life can end at any moment? Am I ready?"