Long QT syndrome is a channelopathy condition which is caused by malfunctioning ion channels. The delicate balance of ion flow is vital for normal cellular function.
In Long QT Syndrome, mutations cause changes which disrupt the balance of ions (usually potassium or sodium).
Imagine you're hosting a party that starts at 7 and ends at 9 but you accidentally sent some invites out telling some guests to arrive at 8:55.
That's essentially what your mutated ion channel has done. The late arrivals come in and restart the party when it should be winding down.