The actual cause of the Cameroon Line is described by many authors (including the 2 included in this answer as being source of controversy and uncertainty - but it seems that it is not an active rift system, certainly not like the east Africa Rift. According to the article A lithospheric instability origin for the Cameroon Volcanic Line (Milelli et al. 2012) describe the Cameroon Volcanic Line (CVL) as being
an enigmatic structure that defies common dynamic models of
melt generation and volcanic activity on Earth.
The enigma is due to the CVL having no spatial age progression in the rift system, despite having continual magma intrusion over 70 million years along the 1600 onshore-offshore line.
The authors also suggest that
The chain is nearly perpendicular to the coastline and
terminates in a Y-shaped structure that has not been affected by absolute plate motions, implying that
the mantle upwelling that feeds magmatic activity is attached to the continent.
The authors model that rather than a developing rift system, the CVL is likely to be an example of where the lithosphere becomes unstable due to cooling from the crust above.
It should be noted that the CVL lies in between 2 cratons, shown as white in the seismic tomography image below from Macquarie Universty - what also should be noted, is the colour of the active East African Rift.
Further, according to the article Formation of the Cameroon Volcanic Line by lithospheric basal erosion:
Insight from mantle seismic anisotropy (Elsheikh et al. 2014), modelled the sub-crust dynamics that
involves a channel flow at the bottom of the lithosphere originated from the
NE-ward movement of the asthenosphere relative to the African plate.
from this, the authors hypothesise that
thinning of the lithosphere through basal erosion by the flow leads to decompression melting and is
responsible for the formation of the CVL. The model is consistent with the lack of age progression of
the volcanoes in the CVL, can explain the formation of both the continental and oceanic sections of the
CVL, and is supported by previous geophysical observations and geodynamic modeling results.
A key conclusion the authors made regarding the flood basalts and volcanism in the CVL is that observations do not support mantle convection or mantle plume causes, rather, they assert that basal lithospheric erosion due to
concentration of mantle flow associated with the
sharp change in the orientation of the continental margin of western