The U.S. Justice Department describes what constitutes a conspiracy in restraint of trade:
Price fixing, bid rigging and market allocation are among the group of
antitrust offenses that are considered “per se” unreasonable
restraints of trade. The courts have reasoned that these practices,
which invariably have the effect of raising prices to consumers, have
no legitimate justification and lack any redeeming competitive purpose
and should, therefore, be considered unlawful without any further
analysis of their reasonableness, economic justification, or other
factors. For most other antitrust offenses, the courts have
established an analytical approach labeled the “rule of reason.” Under
the “rule of reason,” the courts must undertake an extensive
evidentiary study of (1) whether the practice in question in fact is
likely to have a significant anticompetitive effect in a relevant
market and (2) whether there are any procompetitive justifications
relating to the restraint. Under the “rule of reason,” if any
anticompetitive harm would be outweighed by the practice’s
procompetitive effects, the practice is not unlawful. Virtually all
antitrust offenses likely to be prosecuted by a United States
Attorney’s office will be governed by the “per se” rule.
Establishing that this is an illegal restraint of trade under the "rule of reason" would be challenging for Musk in this case. There are legitimate business reasons that are not anti-competitive to drop Twitter such as the tarnished brand image. Marketing decisions are particularly open to "soft" considerations like that.
It also requires a showing of a conspiracy between the alleged participants as opposed to independent decision making, as explained at the same link, which would be at least challenging in this case:
Conspiracy or Agreement: The conspiracy or agreement to fix prices, rig bids or allocate markets is the key element of a Sherman
Act criminal case. In effect, the conspiracy must comprise an
agreement, understanding or meeting of the minds between at least two
competitors or potential competitors, for the purpose or with the
effect of unreasonably restraining trade. The agreement itself is what
constitutes the offense; overt acts in furtherance of the conspiracy
are not essential elements of the offense and need not be pleaded or
proven in a Sherman Act case.
In a Sherman Act criminal case, general intent must be proven.
Customarily, however, proof of the existence of a price fixing, or bid
rigging or market allocation agreement is sufficient to establish
intent to do what the defendants agreed among themselves to do.
The agreement need not be embodied in express or formal contractual
statements. It must merely constitute some form of mutual
understanding that the parties will combine their efforts for a
common, unlawful purpose. The ultimate success of the venture is
immaterial as long as the agreement is in fact formed.
Knowingly Joining the Conspiracy: To find the defendant acted knowingly, it must be shown that the defendant acted voluntarily and
intentionally rather than because of a mistake, accident, or other
innocent reason. The defendant must have joined the conspiracy with
the intent to assist or advance the object or purpose of the
conspiracy. The government need not show that the defendant knew all
the details or members of the conspiracy nor that the defendant
participated in all the conspiratorial acts. Also, it is not necessary
for the defendant to have participated in the origination of the
conspiracy. Of course, mere knowledge of the conspiracy without
participation in the conspiracy does not make a defendant a member of
the conspiracy. In addition, a defendant who knowingly directs another
person to participate in the conspiracy is responsible for the conduct
s/he directed just as if s/he directly participated in the conduct. If
a defendant is shown to have joined a conspiracy, the defendant is
presumed to remain a member of the conspiracy until the conspiracy has
been completed or abandoned or until the defendant has withdrawn from
the conspiracy.
