That is not true.
This figure is not moving across the grass.
He appears for less than two seconds and does not move from that spot.
There is a suggestion this figure is starting to get up from a prone position, that's all.
I can't understand how you are seeing him moving across the grass when it doesn't happen.
Below are three stills from the Bell film showing this figure when we first see him, half way though and the last moment we see him. The duration of his appearance is less than 2 seconds. In that time he does not move from that spot.
On top of that, he is not stood up, he is lying across the steps and about to get up:



Think it through.
You originally claimed the Wiegman figure was at the top of the steps & on level ground with the Hester's. The Top of the steps would be on-line with the Bench we see on the (R). If this figure with a hat on is Wiegman, he has to go Back Down those steps in order to eventually arrive alongside the Newman Family at the bottom of the Knoll. The Bell Film still frames posted above show Wiegman has stopped filming and gone Back Down those steps. He is now on the Knoll Grass. He has pivoted/turned to his Right/(W), and is stumbling or falling. We are seeing him stumbling/falling from his (L) side. What we are seeing is like watching a football player stumbling/falling (R-L) on a TV screen. And this figure is moving. Just look at his (L) hand. If this is Wiegman above, he eventually arrives alongside the Newman Family on the (L) of the street light pole that we see in these still frames. If this figure is Wiegman, how do you believe he eventually arrives on the (L) of this light pole? This figure has to move to our Left/(W) at some point in time to eventually be on the Left/(W) of the light pole.
The Couch Film shows what is believed to be this same figure moving STRAIGHT DOWN the knoll. This STRAIGHT DOWN the knoll would be happening after this figure moved to our Left/(W) across the top of the knoll. Please familiarize yourself with a good copy of the Couch Film. Stay away from the Couch crappola copies.