I'm a theropod, I'm in water up to my neck (per the picture) and I'm swimming along, my feet occasionally making contact with the bottom of the river, leaving some footprints for posterity.
Can someone please explain to me the mechanism whereby the footprints stay in the mud, the river eventually ceases to flow, the water that was the river evaporates (how long does that take?)-- throughout all of this, the mud still hangs on to the precious imprint of my footprints -- then, the mud bakes in the sunlight and eventually becomes rock, and millions of years later people look at the rock and say "Aha! This was once mud, about 8 ft underwater, and a theropod stepped on it and left a footprint!"
I've been in rivers, streams, lakes, and ponds. I have stepped in mud. I have left footprints. They last less than a minute. Not a single one of my footprints in an underwater environment has yet become a fossil. What am I doing wrong?
You don't weigh a gazillion pounds? ;-)
The key to most fossilization is burial prior to disturbing or eradicating what is to be fossilized, be it body parts, impressions, trackways, etc. In most cases, these sorts of impressions/fossils occur in the mud flats along the sides of river, or in shallow, very slow flow waterways (hence the mud, not sand) or lakes. These were most likely in fine-grained mud, indicting slow to no flow in the water, and were not disturbed (i.e. washed away) prior to burial by an influx of new sediment (i.e. there was a flood, carrying with new sediment to cover the old sediments and anything in or on the surface).
So, most likely these were in some sort of shallow oxbow lake adjacent to a river, or were in a lake bed that was fed by a stream/river. The drag marks on the bed of the lake or river were covered over with sediment prior to being washed away or eradicated by other mechanisms.
Trace fossils are actually quite common in some sediments, particularly in ocean sediments.