Here is an actual full, useful video of what happened. The GIF in this thread starts too late.
Basically, Frank Clark pushes his O-Lineman right through Rivers, with the result that Clark incidentally lands on/slides over Rivers as the conclusion of the sack. I don't see anything "dirty" here, just Clark overpowering, frankly embarrassing, the SD O-Lineman. I can see why Rivers felt he was getting "piled on" by Clark. Rivers purposely then grabs Clark's jersey and pulls Clark back in, to cause a scene, instigate a confrontation, and probably to bark at Clark.
The SD linemen are so embarrassed by Clark shoving them around that it's clearly maddening to them.
[youtube]CHCrwAujWEE[/youtube]