I cant speak for everyone of course but I do think he was cruising the streets for quite awhile looking for a vulnerable target. He may have even seen Sierra previously and knew she was alone at that time of the morning. I guess technically it would be considered random since he did not know his victim nor did she know him but she was the type of victim he was going to target.
I am not sure it was random even though LE said there is no connection between Sierra and the suspect. I do feel she was targeted to be specifically kidnapped because she was alone at the time with no one around to see or hear anything. He most likely would have targeted any young victim who he saw alone.
When the three other attempts to kidnap failed, I believe at that time he changed his strategy and MO and decided to look for a victim who was younger and in an area where the victim was alone. So, imo that is the type of victim he looked for and was going to target for his next kidnapping victim, and it happened to be Sierra.