Is secession treason?

No. Voting on Texas independence is not treason, and the people who throw the word around almost never know what it means. Treason is one of the few crimes the Constitution defines precisely, and a peaceful vote does not come close.

The Constitution defines treason narrowly, on purpose

The Framers had watched tyrants use "treason" as a catch-all charge against anyone who crossed them, so they fenced the crime in. Article III, Section 3 says treason "shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort." That word "only" is doing deliberate work. James Madison explained in Federalist 43 that a tight constitutional definition was the barrier against "new tangled and artificial treasons" used by factions to wreck their rivals. Treason is not a synonym for "a political position I dislike."

A ballot is not "levying war"

Casting a vote is not waging war unless you are prepared to argue that marking a ballot is an act of war, which is absurd. A peaceful, lawful referendum, initiated by legislation and decided by the people, involves no force and no violence. There is no army, no insurrection, no attack. There is a question on a ballot and Texans answering it. None of that is levying war against anyone.

There is no enemy here to adhere to

The other half of the definition is adhering to "their enemies," giving them "aid and comfort." In constitutional terms an enemy is a nation or force the United States is at war with. Voting for Texas independence does not aid a hostile foreign power, because no hostile foreign power is involved. Texas is not a foreign enemy. Texans deciding their own future are not giving aid and comfort to anyone's enemy. The element simply is not there.

Treason is an individual crime, which makes the charge collapse

Treason is something an individual can commit. A state cannot commit it, and neither can an electorate. To call an independence vote treason, you would have to say that millions of ordinary Texans who marked a ballot are each individually guilty of a capital crime. State the accusation plainly and it falls apart on its own. No prosecutor would bring it, because it is not what the word means.

Texas insists on the peaceful, lawful path for exactly this reason

The whole approach is built around law and consent. There is a federal statute that bars advocating the overthrow of government by "force or violence," and the answer is in those words. Texas independence, initiated by a legal process and ratified by a vote of the people, is neither force nor violence. That is precisely why the movement insists the path must be peaceful, lawful, and political. Far from being treason, it is the opposite: a free people using the tools of self-government exactly as they are meant to be used.

The bottom line

Secession, in the sense of a peaceful vote for Texas independence, is not treason. Treason has a narrow, written definition, and a ballot does not meet any part of it. The charge is an insult, not a legal argument.