But at least we have an explanation of sorts now:
[T]he connectivity issues are due to large-scale brute force attacks to wp-login pages. These attacks are overloading affected servers and the fix being applied will limit the rate these attacks are hitting wp-login pages. In addition to the fix that’s being applied everywhere, we’re also mitigating the attack by blocking IP addresses all around our data centers.
WordPress.com users are presumably already protected from this vandalism.