应该是 get knocked down
get + 动词的过去分词 表被动语态
knock down 是一个词组,意思是：击倒,撞倒,(投篮)命中
get knocked down 意思是：被击倒、被撞倒
例如：I get knocked down,but I get up again.
其它的例如：
1.窗户被弄坏
—How did the window get broken
—No one knows.
2.人被枪击
The police say the man was shot when they found him,but they don’t know when he got shot.