If this guys is the perp, he could have based his selection of victims based upon the fact that they were probably in "Condition White" as evidenced by the fact that they were running through an isolated section of the park while wearing headphones, and probably oblivous to any potential threats. I run through fairly deserted parks (and questionable neighborhoods) while wearing an MP3 player with headphones, but I always keep the volume relatively low, and pay close attention to my surroundings - And, I'm a 225 lb guy with martial arts training, not a 110 lb woman. Women running alone should avoid that kind of danger area, especially when they are tuned out wearing headphones.
Or her ring for that matter.
Maybe the police should be looking for someone from the RIAA. Perhaps Chandra had copied music from a CD, or had --gasp-- mp3s!!!!
I don't think this guy is the perp. But, in a possible answer to your question, maybe he heard someone nearby, got scared, and fled.
As an aside. Supposedly she had a knapsack that was recovered. Why would she take a knapsack? Lunch? Picnic? Maybe there are items from the knapsack that could help. More than one eating utensil perhaps? As far as I know, the police haven't said what if anything was in the knapsack.
Which is why he was able to run up behind them without being heard, a fact that no doubt entered into his choice of victims.