1) Lower demand because of weekend.
2) A fortunate combination of a lot of wind at the same time of solar beginning to produce a meaningful amount of power (still early in the year).
3) Not enough storage, and not enough demand shifting.
4) Lot’s of classic power generation that can’t be switched off (mainly nuclear in Belgium and France)
5) Not enough interconnection to other parts of Europe. There is a lot of interconnection, but not enough for this situation.
See the graph below showing for the next few days the expected demand (highest line) and wind and sun estimated production (lower 2 lines) in Belgium:
opendata.elia.be