I think that there's an occasional glitch on Uber's end. I actually accepted one of those trips which disappeared as I was heading towards Kenmore Sq. Then I accepted and locked in a second one, and I had some people wave me down just before I reached the pin. I thought maybe the address was off, but turns out Uber double booked me. The trip that vanished on my screen was still active for that passenger, even though I had a new passenger logged on my end.