Why does a pollutant end up multiplied as it travels up the food chain from producers to consumers?

This is due to bio magnification.

Explanation:

When there is a gradual increase in concentration toxins(pesticides) in the tissues of organisms progressively in the food chain,the term for describing this is called -bio magnification.

The progressive multiplication in the concentration of these toxins may be due to  non-biodegradable nature of the toxins,which are absorbed into the tissues of mostly the producers from the environment faster than they could metabolized by the body.

.Therefore these could not be removed by the producers, during excretion and respiration,and hence build up progressively upwards in the consumers.