Trend of postdoc in theoretical computer science
This is a general trend throughout computer science. Part of it comes from a general gradual inflation in cultural expectations. I suspect that a larger amount is due to the fact that CS is no longer a rapidly growing field. For a long time (several decades), as the field expanded and matured, universities were staffing up their CS departments, and could absorb lots of graduates as professors. Now, though, the great expansion is complete in most developed countries, which means there is a more competitive market and a need to absorb graduates in holding patterns. The field has not yet adjusted well to this expectation, however, in my opinion, and does not have the well developed methods for matching graduates with postdoc positions. This, in my opinion, is one of the major challenges facing CS academia at the moment.