It happened incredibly fast

Polling did see it coming. But the growth was so unprecedented, the pollsters questioned their sanity (read: methodology, or at least sampling bias)

The campaign propelling him happened during the two weeks preceding the election. Enough to create a wave of enthusiasm, just short enough to fly below the radar (of him properly getting scrutinised)

Romania's NSA report suggests the campaign was a copycat of campaigns in Ukraine, Georgia and Moldova. Romania either had an X factor that made this one succeed, or the state actor managed to nail the way it calibrated the campaign this time

A couple of other possible explanations: 1) People voted who don't normally vote, the main reason Trump has often overperformed polls. 2) Pollsters wanted to avoid a preference cascade.

I think 1 is the most likely reason. It's important to remember Romania is much poorer and more corrupt than the typical Western European country which means the established political parties are both less popular and more vulnerable to disruption by (in this case somewhat unhinged) outsiders.

Don't forget: people tend to lie in polls if their real vote preference is deemed unsavoury by the current mainstream media.

This wasn't the case here, CG wasn't a well-known candidate with a large party behind him.

Yes, that's also a very plausible explanation