For some people and of course cat owners/carers it is difficult to simply accept the fact that cats like people, can easily get stressed particularly when they are not feeling well themselves and as result to start behaving differently than normally. Although most cat owners seem to understand and accept that some situations such as for example when there have been significant changes in their cat's environment can lead to changes in the normal behaviour of the cat, such as sudden toilet habit changes, spraying, scratching, etc. But when the behaviour suddenly changes and a friendly cat starts behaving aggressively towards the other cats in the household and in some cases towards the human and even their playing behaviour becomes a bit 'rougher' than normal then it is often quite hard to accept and believe that this behaviour can simply be the result of an illness and often the cat is being accused of being 'unreasonable', crazy or vicious.
But this sudden change in the behaviour could simply be the result of something quite understandable and normal, a possible illness such as earache, paw or some other part of the body itchiness or discomfort, or even something more serious...
We seem to forget that when we do not feel well ourselves and some part of our body aches we more than often change to very grumpy and easy irritable persons and we can be quite rude and sometimes aggressive towards other members of the family who might happen to be around. Therefore it is quite understandable when our cats start behaving in the same way when they are not feeling a hundred per cent well. As they don't have the opportunity and knowledge to know or even being able to ask us to give them some form of medication to relieve the pain they will then do what they can themselves to relieve the discomfort they feel which means quite often just trying to find a quiet place to hide till they are feeling better or in some other cases and when they are other cats/pets/humans around by being aggressive towards some other member of the family who is close by, normally at another cat, someone that they can easily pick on without fear of repercussions, a displacement activity. So quite often when two cats who until now they were seemingly OK with each one's presence start having rather rough fights and one seems to chase the other for no apparent reason it is quite easily be that he is either annoyed with somebody else, another cat/pet/human to whom he cant get to direct or also quite possible simply because he is not feeling that well himself. Sudden aggression therefore should be treated not only as a behavioural problem but also as a result of physical discomfort and it should always this factor taken into consideration and the cat if required should be checked by the vet as soon as possible.
Of course on the other hand consideration should also be taken for the other cat, the 'victim' as this form of sudden aggressive behaviour from the other cat can easily end up creating a very uncomfortable and stressful situation for her which could quite possibly might end up to a physical and psychological discomfort situation. The cat who is being chased therefore it is quite possible to begin showing sighs of such a physical and psychological discomfort (for example she might end up start spraying, visiting the litter tray frequently, scratching and overgrooming, etc as well of developing a sudden fear and refusing to come out of the hiding place or even wanting to come downstairs to eat, etc.)
So it is imperative that any forms of such sudden and unexpected aggressive behaviours are not allowed to become a common thing, or a habit and the cats are wherever possible kept away from each other and prevent the fights or chasing from happening by distracting the bullying cat while at the same time a relaxed environment is being provided for both cats and the cause of that sudden aggression is found and treated as soon as possible.