Decay heat from halogens is the cause of the concern (most FSARs have information on charcoal filter loading). The real concern is that too much decay heat can increase the charcoal tempberature such that the trapped radioactive materia is released.
Per the Nuclear Air Cleaning Handbook
https://hss.doe.gov/NuclearSafety/ns/techstds/standard/hdbk1169/Protection of Carbon-Filled Adsorption Systems
To prevent loss of confinement for radioactive iodine and iodine compounds, carbon-bed temperatures must be maintained at a level where impregnants and trapped radioiodine cannot desorb. This requires the bed(s) to be large enough that specific loadings of iodine cannot exceed 2.5 mg/g of carbon, and that airflow through the bed can be maintained at some level in excess of 6 (preferably 10) linear fpm. If bed temperatures can be maintained below the level where desorption of impregnants and trapped radioiodine takes place, carbon ignition is unlikely. If a fire should start, however, total flooding or dumping of the carbon into a container of water is the only effective means of extinguishing a carbon bed fire that is known at this time. Carbon dioxide and gaseous nitrogen are ineffective against activated carbon fires because the fire feeds on the oxygen adsorbed in the pores of the carbon, and the quantity of liquid nitrogen required to provide effective cooling would be unavailable in most cases.