He was selling weapons to enemies of the US.
Earlier in the film, we see Trask trying to sell the Sentinels to communist leaders, after he was turned down by the US military. This is the meeting where Mystique attempts to assassinate him.
Days of Future Past takes place
just after the Cuban missile crisis (seen in First Class), and the US aren’t particularly friendly with the communist states. It’s clear how dangerous the Sentinels are, and the threat they would pose to any human-based military, so you can see why the US would frown upon these talks.
Trask was discredited by the malfunctioning Sentinels, and a deleted scene “Sentinels Cancelled” confirms that the US government were aware of these talks. As Trask is picking apart the wreckage, a military officer says to the President:
The major over there [Stryker] has informed me of some very interesting intel on Trask. Apparently he’s been selling his weapons to the enemy: the Russians, the Chinese, the North Vietnamese, you name it.
The President cancels the Sentinel program, and has Trask arrested.
This is also backed up by the article, the full text of which is as follows:
Trask Arrested For Selling Military Secrets
WASHINGTON – Dr. Bolivar Trask, scientist and CEO of the Trask Corporation and creator of the Sentinel Program, was arrested yesterday and is being held without bail on charges of conspiracy against the United States Government. Among the list of accusations were: selling protected US property and intelligence information to the Russian Army, the Chinese Government and the North Koreans.
The technological advancements seen in recent years by Dr. Trask’s Sentinels astounded Americans yesterday when they were introduced at an event which was supposed to be President Nixon’s statement of security: however, when widespread pandemonium broke out as a result of his machines malfunctioning, the question arose: can the American people trust machines to protect their nation. Amidst these questions, this investigation into Trask’s business dealing unearthed his associations with enemies of the United States.
Source: the 1080p iTunes copy of Days of Future Past, which is sharp enough to easily read the newspaper.