As there is no sensory input from reality which could cause these sensations, are they technically proprioceptive hallucinations?
Not really, although I think it wouldn't be completely inaccurate to think of them that way. A hallucination has no basis in reality (e.g. auditory hallucinations); what you're describing has a physical basis in reality. It is based on a physical disconnect between your previous reality (a mildly turbulent flight of significant hours duration) and the removal of such stimuli (your disembarkation) while your sensitized vestibulo-occular mechanism continues to function as if you were on board. First noticed on ships, but expanded to flights and other stimuli, it's been called landsickness [1] when of short duration, and Mal de Debarquement if it lasts longer much longer than 1-2 days.[2]
To understand this, a bit of basic physiology is in order. First, there are three semicircular canals located in little pockets of bone called the inner ear, one on each side. The three canals are oriented at (basically) 90° angles to each other so that we can maintain our balance no matter which position our head is in. These are also connected to one's ocular reflexes. Together they're called the vestibolo-occular system, which functions as a reflex (VOR).[3] Because the connections are excitatory, increased firing from a canal afferent leads to contralateral movement of the eyes (which is how spinning ice skaters can keep track of / focus on where they are visually). Normally all this occurs without our notice.
The latency of action of the rotational vestibulo-ocular reflex (r-VOR) is 7-15 milliseconds, which is the time required for the eyes to respond in an equal, but opposite, manner to the motion of the head. This time is remarkably fast compared with the latency for visually mediated eye movements, which is longer than 75 milliseconds.[4]
What happens is that with excitation (say, spinning in one plane), the cilia of the hair cells (see @Chuck Sherrington's diagram here) are displaced, and remain displaced as long as one is spinning with the same velocity and direction. Once the spinning suddenly stops, the cilia proceed to move to their default position, tricking the VOR into moving the eyes in the opposite direction repeatedly until the cilia come to a complete stop.[3] You can observe this phenomenon by looking into the person's eyes immediately after they stop spinning (make sure they are safe from falling.) Once things settle down in the inner ear, the vertigo (perception of spinning) stops shortly thereafter as well.
Flying or being on a ship causes multiple VOR inputs and adjustments (by our brains) to this movement. As the flight continues and you experience choppy motions, your VOR "adjusts" to these frequent shifts and you don't feel them as much. When you disembark, however, there is an adjustment period to go through; we are still receiving mixed signals until we are completely sensitized to the new "normal". It is of varying intensity and duration, and can appear suddenly (maybe on head movement or rapid eye movement) and be persistent or transient.
If you can relax, you can enjoy these tricks the brain is playing on your mind, and marvel at how well we are adaptable. I had my first truly noticeable episodes of this phenomenon while traveling on the vaporetti in Venice for a few days. If you feel it's worse that what I describe, I would start my search with sea legs, landsickness, and MDDS (though the latter is highly unlikely).
[1] landsickness
[2] Mal de Debarquement Syndrome
[3] Anatomy and Physiology of the Vestibular System Gizzi, et. al.
[4] Vestibuloocular Reflex Testing - Amin, et. al.
[5] Baloh and Honrubia's Clinical Neurophysiology of the Vestibular System
By Robert William Baloh, Kevin Kerber, MD
