In general, lead pipes are protected from water by a coating of scale that has built up over the years.
However, a change in water quality (or work done on the network) can upset the chemical balance and erode this protective barrier, allowing more lead to leach into the system.
A good way to deal with this issue is to use a silicate-based additive that acts as a corrosion inhibitor.
Old pipes can corrode over the years; if you introduce a change in the chemical composition of the water supplied (e.g. more acidic) or if you change water sources, elements like lead, iron, limestone and bacteria can detach more easily from the corroded pipes and flow through the network. Those elements will also consume a large portion of the chlorine in the water.