It is the scratchpad RAM. There was a mod on later boards that minimized it. There is a similar video garbage that is sometimes visible on the right edge of the screen that is the same thing. Basically, in the Williams games, the video is a rectangular block of RAM that sits in the middle of the address space. The video circuity reads this RAM and pushes it out as pixels on the video display. When the engineers designed it they assumed that the developers would keep the RAM around the video block cleared so that if there was an overscan it would appear black. Silly engineers , the programmers found out about it almost immediately and started using it as scratchpad space. So when the video circuit underscans it (the top line sparkles) or overscans it (the right edge sparkles) that is just the border RAM being used as scratchpad by the game.
@yellowdog
Thanks for the explanation... always interesting to hear the back story!
Being an engineer who writes sw myself I'm not sure who to side with