What are the reasons that Darnay's frequent trips to France result in a charge of treason in Dickens' A Tale of Two Cities?

In an ironic twist of Fate, whereas Doctor Manette is
first able to get his son-in-law released from his charge of treason against the state,
Manette unwittingly figures into Darnay's recapture and imprisonment.  For, at first Dr.
Manette is heralded as a heroic survival of political oppression in the Bastille, but
upon his second efforts for Darnay, Manette is recognized as the physician who suffered
by the hands of the Evremonde twins, one of whom is Darnay's
father.  

In Chapter 4 of Book the Third, Manette appears
before the court and Darnay seems at the point of being released,
when

the tide
in his favour met with some unexplained check, which led to a few words of secret
conference

and Darnay, ne
[born] Charles d'Evremonde, is returned to prison.  The secret conference that has
been held involves the connection of Dr. Manette with the brother of Madame DeFarge, the
young man mortally wounded after defending the honor of his sister, Therese DeFarge. 
Unbeknownst to Manette and Charles d'Evremonde (Darnay), Madame Defarge has knit his
name into her deadly cloth and has recognized Charles after his having come to speak on
behalf of the tax collector for the Marquis d'Evremonde, Monsieur Gabelle, who wrote him
a desperate letter.  She is in possession of a letter that Dr. Manette has written while
he was in prison ( 1757-1775) that describes the atrocities committed by the Evremonds
against her family.  And, as the son of one of the perpetrators, Charles
d'Evremonde will be held accountable.

In addition to the
vengence which Madame Defarge seeks against this aristocrat named Charles d'Evremonde,
the bloodlust of the revolutionaries has also risen and the "new era has begun."  The
Reign of Terror has begun: 

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The deluge rising from below, not falling from
above, and with the windows of Heaven shut, not
opened!