Descenders

Glitched. I'm stuck at 73%. Have done both on my overall and during a run. Issues with 50k achievement as well.

Edit: This has been patched as of 06/01/19

A more appropriate description of this achievement (at least at the time of my writing this) should be "get 10k rep in your first session". Also note that it seems you have to do this without advancing past the highlands.

I was able to unlock this by simply

1)deleting my local save and stopping the game from syncing with my cloud save

2) scoring the 10k in the first session. No continues, no quick restarts.

3) after i passed the 10k mark (i was at 10070 precisely), i purposely crashed my bike to end the session.

The achievement popped on the rekt screen.

Rep is scored in a few ways:
Air time
Rep is scored doing certain tricks, jumps, and other various skilled maneuvers. Landing tricks (such as backflips and 360s)
Maintaining high speed (of over 45 to 50 kmph)
Sliding your bike (with the right thumbstick)
Nearly missing objects
Completing mini-boss jumps (only found on some tracks with a lot of stunts)
Wheelies
Stoppies
Fakies (riding your bike backwards)
Big Drops (landing a large jump downward in elavation)
Clean Landings (landing on both wheels flat pointed in the direction of momentum)

As you score more points by pulling off tricks and such, you will build up a multiplier that goes from 1x to 3x. Keep an eye on that meter and keep it at 3x by doing small bunny hops in between jumps or other good opportunities to score rep. Spins are the safest tricks in comparison to front or back flips. If you land your bike sideways or backwards, quickly bunny hop again and try to straighten yourself out.

With practice you can easily score 2,000 to 4,000 rep on the beginning tracks in the Highlands. Also be on the lookout for the danger zone tracks and helmet cam tracks as they will award more rep for tricks.