Baker's Cyst (Popliteal Cyst): This is a fluid-filled swelling that causes a bulge and a feeling of pressure behind the knee. It's common in adults and can be caused by a variety of factors including knee arthritis, a knee injury, or knee surgery, but it can also occur without any known cause.

Muscle Strain or Pull: The muscles at the back of the knee can become strained or pulled, leading to pain and pressure.
Tendinitis: Inflammation of the tendons around the knee can cause pain and pressure, especially at the back of the knee.
Hamstring Strain: Although more commonly associated with the back of the thigh, severe hamstring strains can refer pain to the posterior knee area.

Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the leg can cause swelling, pain, and pressure, including in the posterior knee area. This is a medical emergency.
Popliteal Artery Aneurysm: An aneurysm in the popliteal artery, which runs behind the knee, can cause pressure and pain due to its expanding size or if it ruptures.
Neurological Conditions: Such as a lumbar disc herniation or peripheral neuropathy, which can cause referred pain to the posterior knee.

Ganglion Cyst: A lump under the skin that occurs when fluid accumulates in a sac, which can occur at the back of the knee.
Lipoma or Other Soft Tissue Tumors: Benign tumors that can grow in the soft tissues behind the knee, causing pressure and discomfort.
Infectious or Inflammatory Conditions: Such as abscesses, bursitis, or infections that can cause acute onset of pain and pressure behind the knee.