  The recent massive measurements of pulsar frequency second derivatives have
shown that they are 100-1000 times larger than expected for standard pulsar
slowdown low. Moreover, the second derivatives as well as braking indices 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 strong correlation of $\ddot \nu$ and $\dot \nu$
either for $\ddot\nu > 0$ (correlation coefficient $r\approx0.9$) and $\ddot\nu
< 0$ ($r\approx0.85$), and of $\nu$ and $\dot\nu$ ($r\approx0.7$). We interpret
these dependencies as evolutionary ones due to $\dot\nu$ being nearly
proportional to characteristic age $\tau_{ch}$. The derived statistical
relations as well as "anomalous" values of $\ddot\nu$ are well explained in the
framework of the simple model of cyclic evolution of the rotational frequency
of the pulsars. It combines the secular change of $\nu_{tr}(t)$,
$\dot\nu_{tr}(t)$ and $\ddot\nu_{tr}(t)$ according to the power law with
$n\approx5$ and harmonic oscillations of 100--1000 years period with an
amplitude from $10^{-3}$ Hz for young pulsars to $10^{-10}$ Hz for elder ones.
The physical nature of these cyclic variations of the rotational frequency may
be similar to the well-known red timing noise, however, with much larger
characteristic time scale.
