Someone ID'd them as US Citizens & targeted them. Guess this will all come out in the investigation.
Very unsafe to travel today, especially young women.
More likely they were just in the "wrong place at the wrong time". Having lived there in Mexico for several years I am familiar with the area they were traveling in- it is a highway between two cities that is sparsely populated (large ranches and farms in the area) and a likely place for such a thing to happen.
Also the pastor had a good sized church in their city and they may have targeted him based upon that issue alone.