The Dink Network

My possibly completely wrong guess from cursory reading of the source code is that it is because of the screen-border checks on pill_brain.

    if (spr[h].y > (playy - 15))

    {
        changedir(9,h,spr[h].base_walk);
    }         

    if (spr[h].x > (playx - 15))

    {
        changedir(1,h,spr[h].base_walk);
    }         

    if (spr[h].y < 18)
    {
        changedir(1,h,spr[h].base_walk);
    }         

    if (spr[h].x < 18) 
    {
        changedir(3,h,spr[h].base_walk);
    }      

So, a sprite near right edge it will go down-left and sprite near bottom edge it will go up-right.