Obviously I'm no expert in Romainian voting procedures, but how does that make any sense without there being vote tampering? The polling must have been able to see him coming. It is hard to see even a state actor successfully pulling off that sort of insane last minute blitz without resorting to a mind control ray.
Legit or not, something insane just happened.
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
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.