That is really curious, two more this morning on top of the three lost yesterday... like wow!!!? It's not completely clear what was responsible for these sudden downings of aircraft.
The obvious one is the patriot system used in an offensive way, possibly they've been quite smart with how they've used it and not just kept it in a traditional defensive role but taken the risk of moving a launcher right up near the front lines to maximise the range and some trickery/coordination with other systems to locate targets and only give away the position momentarily before scurrying away.
Possibly they have a couple of F16s already but they're unannounced as they've not yet had the bulk of the delivery due?
Maybe some third thing - some other air defence system or some new ability to equip current Ukranian fast yets with advanced air-to-air missiles?