1) Unsold tokens are burned - the most common option used by ICOs. If tokens are burnt, token holders share will increase, therefore its price.
2) Unsold tokens are not burned - founders keep them. It's not a good option at the eyes of the token holders. It can be used as a personal interest.
3) Unsold tokens are airdropped - A few ICOs (eg. FundFantasy) used this strategy. As far as they see it, the token price will be decreased but will be readjusted because people will use it.
Options #2 and #3 are unethical as it is like cheating the investors who hard participated in the ICO. If there are unsold tokens, then they should be either burnt or distributed proportionally to everyone who had participated in the ICO.
I think each project has their own plan, and the most often done by many projects is burning unsold coins. it is done to keep the coin in balance with the funds earned. in addition, it is also possible to make the price more stable.