Yeah that's how I feel. The tree was so healthy last year. I didn't expect it to die that easily. The hard lesson for me is that bonsai don't have much energy reserves due to limited root and trunk mass. This means the tender period of budding must be carefully managed.
Yep. Late frosts and freezes are a big deal, since those happen at the same time trees are budding out. Late winter/early spring is a bitch. We had a forecast last night that temps were going to remain above 40 overnight, but I was cautious. At 11 pm the National Weather Service issued a frost advisory for my area. Brought everything into the basement for the night. This morning at 6 it was 40 F, but with frost.
A couple of years ago, I lost a great shohin sized Bald Cypress from Zach Smith to the same short freeze conditions that killed your oak. Had warm temps pushing the BC to begin budding out in late Feb., then in late March, temps dropped into the 20s for a few days. BC was under mulch, but the exposed top was killed. Thebuds weren't all that far along, only in the "knob" stage too. I thought it would be fine in its protected area...
That one hurt. Was working into a very nice tree.