They might be disasters, scandals, a book/film that ‘everyone’ is talking about. They are crucial things for policy entrepreneurs to exploit, but to do so you need resources, luck, timing, determination etc.
Thomas Birkland‘s work on this is seminal.
“jarring and sudden… become attached to particular problems, providing powerful impetus for action or change” (Jones et al. 2016:15)
Jones, M. Peterson, H., Pierce, J. Herweg, N., Bernal, A., Lambert Raney, H. and Zahariadas, N. 2016. A River Runs Through It: A Multiple Streams Meta-Review. Policy Studies Journal, Vol. 44, pp.13-36.
See also critical events
Highly publicized events are critical triggers of institutional transformation (Fligstein 1990, Sewell 1995, Hoffman 1999). Such public occurrences, here called critical events, are contextually dramatic happenings that focus sustained public attention and invite the collective definition or redefinition of social problems (Pride 1995).
Hoffman, A. and Ocasio, W. 2001. Not All Events Are Attended Equally: Toward a Middle-Range Theory of Industry Attention to External Events. Organization Science, Vol. 12, (4), pp.414-434) p.414