Well, because the guard tells you to. If you're asking why the guard tells you to take your pajama off, it's because in the Amrit Sanchar, the rules of Sikh uniform are enforced. If, for some reason, you arrive with your beard tied up, they will make you parkash your beard.
It should be noted that if, for some reason, you have to appear at the Akal Takhat, you may not appear with a tied-up beard.
In this topic, I don't really want to get into whether you should parkash your beard all the time or whether you should wear bana all the time (without pajama), but I am merely noting that these rules are enforced at Amrit Sanchar, whether by the Nihangs or by SGPC.
Taking off your pajama also functions to verify that you have an appropriate kachera on. This Gurmat maryada is sexist. Women do not take off salwar. Only men are forced to to do so.
It seems it is because this is a University based "research"
Both Hundal and Sian are both "University lecturers". One is a sociology lecturer at York and the other is a journalism lecturer at Kingston.