It's been happening for decades. The media has learned that mass killings sell papers and advertising. That in turn, focuses attention on the mass killings. In addition to that, there are millions more people now than 30-40 years ago, allowing for more nutjobs as a total of the population. For example, how many are aware that there were this many killed + wounded in:
1984 (47)
1989 (54)
1991 (58)
1993 (56)
1998 (49)
1999 (98)
2007 (83)
2009 (75)
2012 (135)