You and Toph were struggling to keep the old library up in the Si Wong Desert so it wouldn't fall into the sand, and it made it extra harder since it was curtailing you and her abilities.

After everyone had escaped the library safely, suddenly, Appa wasn't there. Supposedly he was captured by sandbenders, which made Aang upset, to the point that he went in the Avatar State.

Seeing his glowing arrow, the air flowing around him, etcetera, seemed terrifying. You've never saw him this mad.