When a die strikes a coin, the metal flows into the devices and across the fields. Over time, this metal flow erodes the die, just like a flowing river erodes the Earth. The erosion leaves tiny rivers in the die and those rivers get transferred to the coin's surface as lines. As the die wears, the erosion becomes worse making the lines more visible on the coin. Sometimes they can even drag the device's edge towards the rim of the coin. That's what is happening with yours.