What might be a reason to avoid using the side of the bucket while filling a trench?

Using the side of the bucket while filling a trench can lead to mechanical failure because the bucket is not designed to withstand the stresses imposed by that method of operation. When the material is pushed against the side of the bucket, it can create uneven loads and stress concentrations that may exceed the bucket's design limits. This improper use can lead to wear and tear or even catastrophic failure of the machinery involved.

While there could be scenarios where using the side may seem efficient, the potential damage to the equipment makes it a risky choice. Proper technique not only maintains the integrity of the equipment but also ensures safety on the job site, avoiding operational issues and delays that could arise from machinery problems. Legal considerations and efficiency factors are important but not as critical as the risk of mechanical failure associated with improper use of the bucket.