Is dizziness really unpredictable?

That depends.  If a person has BPPV, the spells are very predictable.  They will tend to occur when a person gets in and out of bed, rolls over, tilts their head back or bends over.   Once the problem goes away, it is difficult to predict when it will return.  Migraine dizziness can sometimes be predictable because of an aura that occurs prior to the spell.  The dizziness following a Migraine is usually aggravated by motion which makes it predictable.  Menieres type dizziness may be predictable based upon the type of diet the person had a few days prior.  After the Menieres attack, the spells that follow are usually motion provoked which makes them predictable. In summary, the first attack of dizziness is usually unpredictable, but the following spells are usually more predictable.  Vestibular rehab can help an individual determine when their spells are predictable.