What causes a G manual review?

This isn't about getting an automatic penalty, this is about someone from G actually looking at your site.
What would cause a manual review?

Copying images that don't have a Creative Commons license, can someone dob you in to G?
A certain level of traffic, say 100,000 hits per month?
Competitor dobs you in for dodgy links?
Grey links which trigger their algorithm for a manual review?

Usually being reported by a competitor who thinks it's best to take out the competition rather than focus on their own site/s...

Though - I've never had it happen to me, and that might just be because I don't do it to others.

A competitor may have reported it or caught yu wile doing some manipulation method in less time. If you are doing regularly then they can lift you from penalty.

I've had a manual review for a site ranked pretty high for a pretty popular keyword. Dont know if it was competitor or what.
Probably a combo of things. Maybe something sets off a redflag but the algo just can't pick up what it is so they check manually.

A few ways to get a manual review:
1) Being reported by a competitor. (numerous times)
2) Spamming google properties. (G+/YT etc)
3) Cloaking/Phishing that's obvious.
4) Trojan/Virus hosting.
5) Trying to rank for a manually audited keyword: think Viagra/Payday loans. Fyi, for those who thinks these aren't manually looked at, you're a bit off. I had 4 websites on payday loans; 1 was top 3 and another was top 8, while the other 2 were coming up; all got a manual penalty as soon as they hit page 1. Roughly a 2-3 week span between top 10 and penalty.