  The measurements of pulsar frequency second derivatives have shown that they
are $10^2-10^6$ times larger than expected for standard pulsar spin-down law,
and are even negative for about half of pulsars. We explain these paradoxical
results on the basis of the statistical analysis of the rotational parameters
$\nu$, $\dot \nu$ and $\ddot \nu$ of the subset of 295 pulsars taken mostly
from the ATNF database. We have found a strong correlation between $\ddot \nu$
and $\dot \nu$ for both $\ddot\nu > 0$ and $\ddot\nu < 0$, as well as between
$\nu$ and $\dot\nu$. We interpret these dependencies as evolutionary ones due
to $\dot\nu$ being nearly proportional to the pulsars' age. The derived
statistical relations as well as "anomalous" values of $\ddot\nu$ are well
described by assuming the long-time variations of the spin-down rate. The
pulsar frequency evolution, therefore, consists of secular change of
$\nu_{ev}(t)$, $\dot\nu_{ev}(t)$ and $\ddot\nu_{ev}(t)$ according to the power
law with $n \approx 5$, the irregularities, observed within a timespan as a
timing noise, and the variations on the timescale larger than that timespan --
several tens of years.
