This regularly happen to other people? Player starts year on the big board in the 60s or 70s and I put no practice effort into then. First 5-6 games they fall slightly down the big board then they out of now where jump to a lottery pick?

Did their ratings change at all (including decreasing)?

My guess would be that their optimal position got redefined. Maybe they were a PF but now are a lottery-level SF?

At the time of the move had only gained 2 stamina and 1 we.

Not unheard of. Not really a glitch per se (I don't think) although it's probably not intended by the original developers of the system; it's just an odd and rare byproduct of how the convoluted system works.

Big Board location is relative to players on other teams at the position the system considers the player's best. When players move up and down a few spots without getting better, it means other players have probably been dropped. This can happen especially on the "cusps" - players right around the projected 1st round cutoff, players down around 90 or so (near where players often won't even be exposed to the draft anymore once the draft has been filled).

Big jumps like this, though, are something else, mostly. The best guess is that the system has started to think of them as a new position - for example, he was the 10th best PF in the country, but after some other teams' player developments, he's now considered a SF, and one of the best in the country. So he's a lottery pick, congratulations I guess. :) I am not entirely sure that's exactly how it works - you can always send a ticket and ask, to see if they'll tell you explicitly why it's happening - but this is the gist of it, I'm pretty sure.

Most movement on the Big Board is gradual and rather slow

Every now and then there is big, non continuous movement.

The only credible explanation I have heard for the big non continuous movement is the "position" change theory - which makes sense to me