[pck24]

Is a driving force necessarily required for the initial formation of the carbocation?  In a large sample of a particular species there will be a Boltzmann distribution of energies in  the molecules.  There will be a particular activation energy for the formation of the carbocation, and in a mixture a small portion of the molecules will have enough energuy to overcome this activation energy (the exact proportion dependant on temperature according to exp(1/T) ).

Once the carbocation has formed then, the nucleophile or the leaving group may reattatch to it forming the product.  I think the driving force only really becomes relevant at this stage of the mechanism, where it is energetically favourable to form a neutral compound.  

Or maybe I'm wrong.

[Winga]

I think it is caused by solvolysis.