乱 + V is a productive pattern meaning 'to V recklessly / without proper basis':
乱说 (talk nonsense),
乱跑 (run around chaotically),
乱花 (spend wastefully),
乱画 (scribble). The complement
一通 (yī tōng) often follows for emphasis:
乱写一通 = 'wrote a whole bunch of nonsense'.