Water running off the mountains does not drain directly to sea, it floods and stagnates.
No major rivers exist, the plant root are too strong to allow it, instead a thick layer of grass and peat covers the muddy ground below.
Digging through it is almost like clay, but more fluid, and it slowly fills in any holes dug for basements, shelter, or food stores.
Lack of trees or rock to protect from the rain force people to seek shelter in pits and trenches to avoid being blown away, only to have the ground swallow them up and seal them in when the mud flows from rainwater.
The few small streams that do exist get cut off suddenly as they disappear underground, the substructure of the land is like limestone, forming sinkholes that suddenly appear and swallow an entire village in less than an hour. Only to be plugged with mud from the next rain.
The land is slightly elevated from sea level on a limestone base (but just 30-100 feet), all access to the sea by river is underground in an unstable catacomb of ever-changing tunnels. from time to time sea swells cause the tunnels to back up and flood with salt water. The tunnels can not be used for storage or shelter. You never know when they will flood, killing people or ruining supplies, or give way and wash you out to sea. Some species of carnivourous eels wash in and become stuck from time to time. They multiply quickly, eating whatever is around, and turn cannibalistic once other food sources are depleted, leaving just one, or a few separated LARGE eels. They may be poisonous, or electric, but it is rare for someone to make it back and try to explain what they saw.
Storms wash fresh water down from the mountains, but after a week it becomes stagnant and kills any human edible crops planted. (If only you could dig ditches in this terrain, someone might one day think of dragging down rocks to line irrigation channels with? Sun-baked bricks may work for a year before crumbling, fire baked bricks 10 years? but the weight of the shifting mud will require large granite from the mountains to stop.)
The stagnant water also brings mosquitoes. They tend to stay around the stagnant water and don't go up the mountain much. It also smells bad enough in some place to make people not want to stick around anyway.
The grass that inhabitants this area is also very sharp and cuts your feet and ankles easy. It stays sharp because the fibers required to live through the rapid wet/dry cycles without being washed away. Even in "Dry" weather you can lay spread out on the grass for only 10 minutes or so without sinking down about four inches before the compressed plant material is solid enough to keep you from getting muddy. You will still stand up slightly damp, even in dry weather. You wont touch the mud unless you start kicking the ground hard, or from jumping or the heavy impact of a good run. It is MOSTLY solid, but never COMPLETELY dry. Shoes tend to stay damp, leading to a nasty case of trench-foot. An extra pair of shoes or two is a must on any excursion. Sleeping on the ground overnight, laying on your back will cause you to wake up with your chest damp, and the back of your clothes thoroughly soaked. (You will most likely not awaken from being well rested, but from choking on inhaled mosquitoes, of the pain of a million bites.
Running here is probably not a good idea. The thick and sharp weeds will cut you deeper if you run faster past them, instead of taking your time to tread them under foot, or clear a path with some type of machete or sickle. The fibrous spines may also cling together like Velcro, this enables them to withstand being blown away in the wind, even though the ground is not solid. The plants hold the ground together, making the mud accumulation possible, and keeping it from becoming just an ordinary elevated swamp.
As ordinary as that strange occurrence might be..
running could also cause someone to trip and fall face-first into the tiny blades. It is considerably worse than a paper-cut, but not as bad as barbed-wire. Not quit as bad.
Any foot-sores or face and arm cuts could become infected by the stagnant wastewater. Any human waste from a nearby settlement would also accumulate here, breeding sickness. If the land were steeper and allowed the rain to escape more quickly to the sea, this landscape would not even exist.
Depending on the local people's level of technology, or understanding of health, they may not even understand this, and just consider it bad luck.