Tirumanjanam is of two types – Lokaanta tirumanjanam and Ekanta tirumanjanam.
Lokanta tirumanjanam is for us to witness; during this Perumal will be adorned with clothes and there is no harm in witnessing this tirumanjanam
It is an offense to see or worship when Perumal is offered the prepared prasadams. He must be allowed to partake prasadam without any obstruction and disturbance. That is the reason no sevakaalam is allowed during this time. When sevakaalam is recited during this time, Perumal will be made to move his concentration to sevakaalam and he will not be able to partake adequate prasadam. It is similar to us watching television while eating taking our focus away from eating.
To facilitate Perumal partake prasadam adequately, other services such as recitation of sevakaalam, prostrating in front of him, chanting, etc. are avoided. Hence we are supposed to witness this; if we do, we will make Perumal focus on us. This is the reason to screen off Perumal while he partakes prasadam.