What causes VFD driven motor bearing current?

There are several things involved, all with varying degrees of impact.

Large machines are - generally speaking - made of pieces (segments) because the circle for the stator and/or rotor core is too large to manufacture from a single sheet. This leads to some breaks in the magnetic flux path symmetry, both in the radial (right angles to the shaft) and axial (parallel to the shaft) directions.

For the most part, the windings of large machines are formed and installed by hand. This too can lead to symmetry issues, as the current paths are not identical which in turn will create some differences in the magnetic field flux.

Output waveforms from power electronics are only approximations of true sinusoids. The presence of additional harmonics distorts the sinusoidal nature and results in changes that are not symmetric in the magnetic field strength ... which in turn means a non-symmetric flux distribution.

Two other items contribute to potentially damaging bearing currents as well. One of these is the Common Mode Voltage which is present (to some degree) in all drives. Essentially this is a signal that is present at both the drive input and output ... I tend to think of it as an offset. It's not something that traditional grounding addresses, and can create an elevated potential in the shaft which then discharges through the bearing path.

A second item is not related to the presence (or absence) of drives at all; it is related to the mechanical arrangement of the process drive train. For example, a shaft that has a sliding seal (like the felt curtain on a dryer section), or one that turns a blade against a gas or liquid (like a compressor) can generate a static charge at the point of contact. If there is no means of isolating this charge to the portion of the shaft where the sliding is occurring, it can pass through to the motor shaft and thence through the motor bearings.

Lastly - the frequency of the variable frequency drive harmonics in the output waveform is significantly higher than line frequency. This requires specific accommodations for grounding as traditional methods are insufficient due to the attenuation caused by the relatively high resistance ground path.