Ответ: В UM на 2138 написано, что:
On the LPC2131/2132/2138, the pins above are multiplexed with P1.31-26. To have them come up as a Debug port, connect a
weak bias resistor (4.7-10 k depending on the external JTAG circuitry) between VSS and the P1.26/RTCK pin. To have them
come up as GPIO pins, do not connect a bias resistor, and ensure that any external driver connected to P1.26/RTCK is either
driving high, or is in high-impedance state, during Reset.
Повидимому, в Вашем Вигглере наличествует резистор с RTCK об землю. Таким образом, когда он воткнут - отладка разрешена, а выдернут (нога процессора висит в воздухе) - запрещена.