track js is altered mid way

I'm a complete new-bee to the world of GPS tracking/logging (is there any difference?) so it might be something That I do with out being aware of it.
i start my tracking and get on my way. at the end I noticed that the first part of the track is straight. today I tested it mid way and noticed that my movements were followed properly but again at the end of my trip the first part was straight from start point to a certain point and then it it was ok.
any ideas? Here is a gpx file of the track and a screenshot of it on the map