Due to interactions between different generators, there can be instabilities causing voltage or frequency or reactive power to deviate outside of spec. A simple example might be two generators where one surges while the other drops back, then vice versa. The measurement (by the network operator) of these effects is poor for Spain - shown by the simple example that they have large oscillations that they couldn't explain.
There's path dependent healing and correction of problems by different generators, which overall leads to network stability. However the network operator here is not actually resolving cause and effect, and does not have the insight to manage their stability properly.
In this case you can see them trying a few things to inject changes that they hope will bring stability - e.g. tying many connections hoping that adding generators together into one network will resolve to a stable outcome.
Are there countries that have a better design for their electricity network control systems?
Disclaimer: I don't design electricity networks nor electricity markets. And the above is ignoring loads (loads are mostly less problematic for control than generation).