The slowdown is the long-term trend caused by drag of the Moon's gravity, but the speedups are sporadic and as a result of events such as massive earthquakes th

The slowdown is the long-term trend caused by drag of the Moon's gravity, but the speedups are sporadic and as a result of events such as massive earthquakes that cause the Earth's rotation to speed up as some significant mass falls further to the Earth's center. The conservation of angular momentum is the principle that leads to those brief speed ups, but they will not fully counteract the longterm drag caused by the Moon. Edit: TLDR; Local maxima vs. overall trend.