When we bend forward, initially, the erector spinae help control our descent. If they did not, we would fall forward suddenly rather than gradually. This is an example of an eccentric muscle contraction, where the muscle is lengthening but doing so actively. 

As we enter the forward bend more deeply and the spine rounds further, erector spinae activity significantly reduces: it is no longer contracting. Whilst this may help us enter a posture more deeply, we lose the protective role the erector spinae had on the spine. As a result, more significant strain and load are placed on the ligaments and intervertebral discs.

As long as the load is not too great, and these tissues are healthy, there is no issue. They are, after all, in place to protect the spine. Furthermore, stimulus like this may have a role in improving these tissues. However, if the load is high or these tissues are already injured, it can cause further damage and worsen the individual's symptoms. 

Shalabhasana (locust), dhanurasana (bow) and Vyaghrasana (tiger pose) all activate the erector spinae muscle group and may help strengthen and improve it.  However, of the three postures, Vyaghrasana is the safest for the spine, provided the spine does not extend as the leg is raised (see video). Extension loads the spine more strongly, so a weak or injured spine may not be able to tolerate it, leading to further pain and dysfunction.