  Monitoring of the narrow line Seyfert 1 galaxy Markarian 335 (Mrk 335) with
the Swift satellite discovered an X-ray flare beginning 2014 August 29. At the
peak, the 0.5-5keV count rate had increased from that in the low flux state by
a factor of 10. A target of opportunity observation was triggered with NuSTAR,
catching the decline of the flare on 2014 September 20. We present a joint
analysis of Swift and NuSTAR observations to understand the cause of this
flare. The X-ray spectrum shows an increase in directly observed continuum flux
and the softening of the continuum spectrum to a photon index of 2.49
(-0.07,+0.08) compared to the previous low flux observations. The X-ray
spectrum remains well-described by the relativistically blurred reflection of
the continuum from the accretion disc whose emissivity profile suggests that it
is illuminated by a compact X-ray source, extending at most 5.2rg over the
disc. A very low reflection fraction of 0.41 (-0.15,+0.15) is measured,
unexpected for such a compact corona. The X-ray flare is, hence, interpreted as
arising from the vertical collimation and ejection of the X-ray emitting corona
at a mildly relativistic velocity, causing the continuum emission to be beamed
away from the disc. As the flare subsides, the base of this jet-like structure
collapses into a compact X-ray source that provides the majority of the
radiation that illuminates the disc while continuum emission is still detected
from energetic particles further out, maintaining the low reflection fraction.
