I don’t believe it’s actually valve ticking the way lifters start ticking in some cars. The noise they are referring to is actually the sound of an exhaust leak that gets worse the longer the headers are installed.
Headers are notoriously difficult to keep sealed up tight. Usually stainless steel expands more easily than the cast steel that factory exhaust manifolds are made of. This expanding cause cracks in any vulnerable joints. There are few reasons why headers can be difficult to keep sealed up tight and this is primarily what keeps many people from ever buying headers twice.