What predicts (causes?) party realignments?

Richard L. McCormick discusses this question in his The Party Period and Public Policy (the quoted chapter is reproduced in jstor):

After coding and analyzing the contents of party platforms and federal statutes, Benjamin Ginsberg conclude that realigning eras were marked by high degrees of ideological difference between the parties and by significant transitions in national policy.  David W. Brady and several coworkers marshaled evidence of heightened party voting in Congress and of the adoption of “clusters of policy changes” following the realignments of the 1890s and the New Deal era.

The simple-minded amongst us might be tempted to conclude we are likely in a realignment period right now, as argued very recently by David Frum.  It should be noted that McCormick criticizes these theories for their simplicity in some regards, and their rather casual aggregation of different time periods.

Here is the Ginsberg piece from jstor.  Here is one of the cited Brady pieces, again jstor.