Why do some players get a 6second bonus every time their clock runs to 0 ?

Could be a correction for lag and it could be that the game's time settings have an increment added to both players clocks after their respective moves.

time is end and he still playing?!!!

Technically the time wouldn't have ended -- if it's lag correction it would have said 6 seconds on your oponent's computer when he made his move, but zero seconds on yours by the time the notification of his move made it to your computer some six seconds or so later.

but it keeps going until I am out of time...maybe 3 or four times.

Your opponent is likely seeing the same thing on their side with your clock only they're moving faster than you -- possibly even using pre-move.

there's lag on the chess.com server,  lag on your computer -- best is to have a wired connection to your DSL line, clear your cache, and don't have any other windows or programs running.    it is annoying during bullet games when you factor in time to your strategy.  you see an opponent go to zero, then go back up 5-10 seconds.  screws ups your game.