This is defined in the law as very bad behaviour, such that it is no longer possible for you to live with them. You could not bear to live with such behaviour. One form of unreasonable behaviour is called physical violence. If your partner has physically attacked, or assaulted you (and if that counts as physical violence legally) then you have grounds for a divorce. A related ground for divorce is that your partner has made insults or threats against you. This counts as verbal abuse and is a very damaging form of abuse, taken seriously by the courts.
Another form of unreasonable behaviour involves forms of addiction. If your partner is often drunk, and abusing alcohol, then it is not reasonable for you to live with them and you may be able to get a divorce. Drug taking also counts as unreasonable behaviour. Often the two may be linked and you may find that this compounds the grounds for divorce.
Finally, if a partner is not giving money for housekeeping then this could count as a reason for divorce. This is a form of unreasonable behaviour which is understood by the courts.
The list above is not an exhaustive list of examples of unreasonable behaviour and there may be many other cases which can be classed under this definition. It is best to consult with us for legal advice.