Some events get emphasised by the media (or politically), and become discursive events, and others don't. For example, Chernobyl WAS a major discursive event, but the similar Harrisburg incident was barely reported at all, and the media even kept it secret for many years. This depends on the respective political power constellation and developments.
In the food and farming area, a similar example is the UK outbreak of BSE, which was (eventually) a major news story wordwide – it was a discursive event, and it has formed a point of reference ever since. There may have been similar outbreaks of animal disease which did not become ‘big news’, and indeed, there may have been major initial efforts to prevent information about BSE spreading. The BSE outbreak typifies a discursive event which had an impact on a very wide range of other discourses worldwide, and it also typifies a key reference point in a number of debates. Some authors use events like BSE as a shorthand in their discourses.
Discourse events can be useful to mark out the contours of the discursive context to which a current strand relates. A synchronic cut through a discourse strand can find its historic roots by looking back at the main discursive events throughout history, and may look at the way the discourse has evolved and changed. A diachronic cut through a discourse would give a cross-section of the range and manner of discourse in a particular strand and sub-strands.