What would cause some hi scores to spontaneously reset?

[Ummon]

I'm not sure this happens with Mame, but it does with Advancemame on occasion.

[Robbbert]

Mame no longer supports high scores because of the all the spurious bug reports it created (so they say). I'd guess it never worked perfectly anyway.

MAME32FX still has it, maybe you could try that.

[dna disturber]

I use a hi-patched mame and never encountered a reset (thus far)......

It can happen if you update frequently to the new mame version (and update the roms), if there is a namechange of the games. 
For instance mario gets renamed marioa because they have a new set that they want to make the parent. Now there is a new mario.
You'll start mario as always but this game is somewhat different in saving the hi-scores (internall adresses don't quite match in hiscore.dat). So it wil become the initial score-board.
But if this would be the case you can't set a hi-score for that game because it will continue to only give the initial score-board.

Maybe this is the case or maybe it is something completely different....(and then I don't know)

Dna Disturber 

[Cananas]

If you don't give us more information... should be one of those:

- The name of the game and the name of the hi file don't match. (the set was renamed)

- Something have been changed (fixed) in the emulation of the game, so the hi file is no longer valid.

- A game does not have hi score support.

- You reset the game (F3)

- A bad hi file data

[Ummon]

Saying Advancemame actually tells you a lot. Cos it stopped at about .106, so still hiscore support, and at that time doing a hard reset didn't empty the savestate score cache or whatever. Anyways, I think I might know what happened. I tried out that new 'roms updated' version of Advancemame (still .106-era engine), which seemed to have some kind of effect.....though, I swear there have beens times where it's just happened. I dunno. It doesn't reset all games. Just several, for example DK.