ED_RecluseSometimes this appears to happen when you leave a gravity well. When you come out of FSD near a star and then you start Supercruising toward a station, it takes some time to get out of the star's gravity well, or the gravity of any close planets. Keeping throttle maxed, you can see your speed/top speed starting to increase and sometimes it is an abrupt jump that seems like a "TURBO BOOST" when you are out of any gravitational influence. There is no BOOST mode in Supercruise so I doubt it is anything to do with your throttle.
If not in SUPERCRUISE and approaching a station, are you going into AUTO DOCK? That will take over and boost to get you lined up with the station for docking.
I agree and thought the same before his response about moving the key bind. We often don't pay a lot of attention to the grav well of the planet as we approach and some planets have more pull than others especially if you approach mail slot from the planet [you know, too line up and get in if you are, ahem, carrying questionable cargo]. One can also experience the 'boost phenomenon' for lack of a better term on planets with higher gravity as you descend/approach a base and the gravity is higher than you notice/expect. I was lost in thought a few weeks a go and not paying too much attention to the decent and approach in my python and caught it in time otherwise I would have been buried to to my corn hole in the side of a crater wall, lol. Gravity...it's over rated :-).
d