Why would a transmission jerk when decreasing speed?

Worn clutch packs in your automatic transmission could do it, tranny fluid level might, and even could be your torque converter. BUT it could also be worn universal joint on you driveshaft if you have a rear wheel drive vehicle because your causing a shift in what drives what when you decrease speed, causing the play in the worn out U joints to snap to the other side of themselves. Check your driveshaft for play before you even start to mess around with the transmission.