But since they have no root, they last only a short time. When trouble or persecution comes because of the word, they quickly fall away. Mark 4:17
But their character is weak. They can't absorb the spiritual truths they need to grow in faith. And so they "fall away."
"Fall away" comes from the Greek root word skandalizo, from which we get the English word "scandal." The Greek word meant to tempt to sin, to cause to distrust, or to place a stumbling block. Make sure you are so grounded that your character is not weak.