Increase in clients with vertigo?

Hi! I am seeing several of my long term clients suddenly becoming increasingly dizzy, or with vertigo. It is getting to the point where I have to give them 30-60 seconds in between posture changes to give their body time to acclimate. Is anyone else seeing this? It just started about 6 months ago, and I just had ANOTHER client start having this issue (they are all of varying ages, all have been long term and never had this before).