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.