This has happened to me on more than one occasion. A JW suddenly notices you at the hall and asks to work with you. This continues for a while and then you are dropped. Reason? They just wanted to get their time in.
I once auxiliary pioneered for a month and took two weeks off work. A brother in my hall who was about my age suddenly took an interest in me and became my pioneer buddy - he would pick me up and drop me off. We'd spend about 2-3 hours out in service. We'd also visit the elderly, housebound sisters and drop the meeting tapes off on the way - easy time.
What really bothered me is that he would talk among his friends out in service about various social gatherings, with me standing there feeling like a lemon, and not once was I ever invited. When I stopped pioneering, he never asked to work with me again.
I've also seen pioneers use Bible studies to get their time in. They would take a study to Bethel and on the way ask him or her questions about the truth to keep their time going.