Why do high voltages develop across the leads of an open CT secondary?

Current Transformers are transformers that produce proportional current in the secondary. The open-circuit voltage of a CT has absolutely nothing to do with the system voltage of the primary. For a given core secondary winding design, the secondary open-circuit voltage is the same, whether the CT is on a 400V circuit or an 110kV system. The voltage developed on the secondary of an open-circuited CT is a function of the current flowing in the primary and the inductance properties of the secondary windings. If the secondary is open-circuited, that is seen as infinite impedance, so all of the primary current is forced through the magnetizing branch. The voltage seen on the secondary is this primary current times the magnetizing impedance.