I guess what has led me to believe it was a crime of opportunity is that it would be much easier (I think) to abduct someone on foot. There are plenty of young women around that time of night, many intoxicated and not fully aware of their surroundings, that would be an easier target. Why not follow and grab one of these girls on their way into their home?
Intentionally hitting a bike rider for an abduction takes a lot if risk. It could draw unwanted attention, you could be seen exiting your vehicle, there could be an undetermined amount of time to retrieve the bike and victim, you would have to control the victim while you fooled with the bike, the victim would at least suffer some injury and may bleed leaving DNA in your vehicle. It is just so unpredictable knowing where the victim and bike will both end up and knowing how much force to use for the desired result.
If this was not an accident then I think it had to just be an uncontrollable impulse to grab her by any means possible.
This all keeps me believing that it was not preplanned and he probably had to make some quick decisions on how he would cover his tracks.