I find it quite irritating that your submissions can be anonymously flagged with no justification given. There’s also no way to dispute the flag.
> On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.[1]
A posting saying some service is down doesn't have any intellectual content. Just because people post these all the time doesn't mean they're on-topic.
Also, if someone depends on a service, they probably don't need HN to tell them the service is down. They can easily check the status page themselves, and the status page will probably be more up-to-date than the HN post.
"Foo is down" posts tend to be offtopic because (a) there are so many of them, and any kind of repetition makes a story less interesting for HN; and (b) as minimaxir pointed out already, there's usually no other information than "Foo is down", so there's no basis for a substantive thread. People then end up arguing either generically about Foo, or they get mad about flaws in the status page or something.
What is interesting and tends to do well on HN are the post-mortems that sometimes show up after an outage, explaining what happened.
Try not to single out “your submissions” or a particular company/entity while trying to find a reason -- there may be no specific reason.
Best of Luck.