Can someone physically explain to me the cause of this nonuniformity
in the field lines?
It is due to the Lorentz contraction. The field of a moving point charge is contracted along the line parallel to the motion:
A very important application of the electric field transformation
equations is to the field of a single point charge moving with
constant velocity. In its rest frame, the electric field of a positive
point charge has the same strength in all directions and points
directly away from the charge. In some other reference frame the field
will appear differently.
In applying the transformation equations to a nonuniform electric
field, it is important to record not only the value of the field, but
also at what point in space it has this value.
In the rest frame of the particle, the point charge can be imagined to
be surrounded by a spherical shell which is also at rest. In our
reference frame, however, both the particle and its sphere are moving.
Length contraction therefore states that the sphere is deformed into
an oblate spheroid, as shown in cross section in Fig 4.