Cats can drool under certain conditions which are considered to be normal. These can be stimulus to food, excitement, happiness and exposure to hot conditions. However, excessive and sudden drooling in cats is not normal and can be due to any of the following factors:
Foreign material trapped in teeth
Ingestion of medicines
Ingestion of flea spray
Ingestion of poisonous small animals
Salivary gland infection
It is worth noting if your cat has had to urinate more often, which is a sign of diabetes, liver or kidney disease. Or if it is opting to eat soft food and leaving harder to eat foods such as biscuits would be a give away that there is a problem with your cats teeth or gums.
Normally keeping the cat under close observation indoors and monitoring litter tray habits can help to detect the cause of the problem.
Do not be too concerned if your cat drools a lot when petted this is considered normal. However, if it is more lethargic than normal. It i recommended to take your cat to vet because treatment is dependent upon the diagnosis of accurate cause.