Graves do have a tendency to create slumps in the ground like that as the soil "deflates" or settles back into the grave. This depression should roughly be in the size and shape of a coffin. This tends to occur if not enough soil was put back into the grave. Under the same circumstances the soil in the grave can "peel" away from the walls in the shape of the grave that was dug. You can see both the slumping and the peeling in this photo.

As for dandelions, I'm not aware of them being specifically planted to mark gravesites, but often cemeteries have different patterns of vegetation than their surroundings. This is true of most human constructions. For instance, in the Southwest, the "Brigadoon effect" is a well known phenomena that helpfully outlines adobe walls through vegetation that refuses to grow on top of those walls. For instance, the very sharp contrast that reveals the rectangular room here. These vegetation changes can be fairly localized (like the Brigadoon effect) or impact a larger area (like a cemetary).

Two big things tend to change vegetation patterning in a cemetery. First, many cemeteries once had fences which can often change the way soil is deposited inside vs. outside the cemetery, as well as how water accumulates in the soil. Secondly, depending on how bodies are buried, the decay of both coffins and bodies can help to change soil chemistry locally that makes them more or less favorable for plant growth. The very specific changes to soil deposition, water availability, and soil chemistry means that certain plants are more apt to colonize the impacted area and grow really abundantly. Just like the Brigadoon effect, this means the plants will often outline the cemetery (at least roughly) because the soil changes are so highly localized. It very well may be that dandelions in particular often enjoy the kind of micro-environment created by a cemetery, but I can't actually verify that.

However, this is all to say that having never seen the site and not having expertise on the East coast, I can't confirm or deny that what you saw was in fact an historic cemetery. Only a professional with expertise in the region who has actually seen the site can do that for you.

Along those lines, as for what you should do, first thing first is to relocate the site if you can. The best thing would be to get GPS coordinates for the site, but I know that isn't always an option. Otherwise, if you are good with a map, getting rough lat/long coordinates and writing down detailed directions to the site would be the best thing to do. Taking as many photos as you can of the important features (the depressions, the vegetation pattern, and the potential grave markers. If you can write out directions to the site by memory that would be the minimal amount you should do.

Then, once you have as much documented as you can, call or email the state archaeologist for the county you found this in. Their contact information is here. Most states have a SHPO (State Historic Preservation Office) that employs archaeologists whose job is preservation of state archaeological resources. However, they are generally first concerned with archaeological resources located on state land (and sometimes federal land). If you found this cemetery on private land they will probably be less inclined to go take a look at it, but they might be able to help given whatever materials you send them (e.g. written descriptions, location, photos, etc.). If they are not interested because of the land ownership, you could ask them to direct you to a museum or university affiliated archaeologist they know who might be interested in helping you out further.