  Recently observed minute timescale variability of blazar emission at TeV
energies has imposed severe constraints on jet models and TeV emission
mechanisms. We focus on a robust jet instability to explain this variability.
As a consequence of the bulk outflow of the jet plasma, the pressure is likely
to be anisotropic, with the parallel pressure $P_{||}$ in the forward jet
direction exceeding the perpendicular pressure $P_{\perp}$. Under these
circumstances, the jet is susceptible to the firehose instability, which can
cause disruptions in the large scale jet structure and result in variability of
the observed radiation. For a realistic range of parameters, we find that the
growth timescale of the firehose instability is $\approx$ a few minutes, in
good agreement with the observed TeV variability timescales for Mrk 501 (Albert
et al. 2007) and PKS 2155-304 (Aharonian et al. 2007).
