This was addressed by the character's actor, Raphael Sbarge in an interview with TVZone. In short it was a combination of upset at Janeway's failure to make a peace with the Kazon, unrequited passion for Seska, self-interest (in thinking that the ship was very likely to be taken by the Kazon and wanting to be on their good side) and blackmail; the threat of exposure if he failed to comply.
"He clearly saw that Voyager was going into some very dangerous
territory without any sort of help. I think he began to feel that they
were surrounded by the Kazon and would eventually be blown to bits
unless they made some genuine and serious moves towards making peace
or creating a balance with them. In his own way, at least initially, I
think he did what he did from a sort of benevolent point of view. It
wasn't completely good for everyone because it was really
self-motivated but I don't think he intended for it to get as bad as
it did. He eventually got himself in a position where Seska was
blackmailing him and then he was stuck. When you lie you have to cover
your lies. One lie leads to another lie and that's what happened to
Jonas. He got in too deep." Regarding Jonas' familiarity with Seska,
Sbarge reckoned, "My sense was that they probably knew each other when
they were young and maybe there was even a romantic thing that went on
there. That was my thought. Whether that is what was in the producers'
minds, I don't know,"
Memory Alpha: Jonas - Background information
Trying to bargain with the Kazon after Janeway failed to secure a deal.
HOGAN: Nothings going to come of all this. B'Elanna, you were Seska's friend. Couldn't you get word to her, try to work things out?
TORRES: No, I couldn't.
Attempting to ingratiate himself with the Kazon.
JONAS: In a matter of speaking. It's all in the file. This should prove my worth to you, Rettick.
RETTICK: We'll see.
A desire to ingratiate himself with Seska.
LORRUM:Mister Jonas, your enthusiasm is appreciated, but I would prefer to know more about it before I bring it to Maje Culluh's
JONAS: If I could just talk to Seska.
Personal threats against him
Seska: One way or another, I'm going to take Voyager. You can either help me, or you can suffer along with Janeway and the others.
Don't worry, Mike. Everything's been planned to the letter. Here's how
it's going to work. We'll be waiting for you on a planet called