There are two very different forms of hashtag hijacking. The first is when an advertiser uses a popular social madia hashtag to place its brand or message. The second kind is when a competitor, consumer or lobby uses the hashtag which has been promoted by the brand to attack the latter.
In the first case hashtag hijacking is a practice by which an advertiser uses a popular Twitter hashtag - or other types of hashtags - to gain visibility on social media. When hashtag hijacking is related to specific events it can be posted from a brand newsroom. It is a classic form of real time marketing.
Examples of hashtag hijacking:
A use of #royalbaby hashtag:
An example of humorous tweet mocking the 2014 iPhone Bendgate :
Another example related to a planned event:
Real-time hashtag hijacking is not without risk. Digiorno Pizza jumped onto the popular hashtag #whyIstayed without looking into its context related to Ray Rice punching his fiancee. The brand inevitably suffered a backlash.
The second form of hashtag hijacking is when the brand is attacked through its own hashtag during a campaign.
A recent example of hashtag hijacking associated with the Doritos #crashthesuperbowl campaign :