Why Won’t Some People Tell Where They’re Going when Changing Jobs?

Good chance new firm/recruiter coached them into not providing any details to protect against possible counter offer or any potential retribution

Either way, no real upside to mentioning anything until you are safe in the seat

He's protecting himself. Don't take it personally. There are vindictive people in the world. Less likely that a firm/PM would try to sabotage a junior analyst, but it happens quite frequently at senior levels. 

My prior firm would bad mouth all departing PM / spinouts to its LPs. Attribute negative performance to the departing PM and spin narrative so that it seems like the firm took corrective action, or deemphasize their contributions entirely to make it seem like they were a back office support staff that assumed no risk. 

Also if your notice period is, idk, ~6months, a lot can happen during garden so its better to keep to yourself.

There are many vindictive PMs in this game. Zero upside in telling your senior where you’re going unless he helped you land it.

Whenever I poach an analyst, I recommend that they avoid sharing the destination with their current employer as long as possible. 

We've found the "vindictive PM" archetype will typically reach out to all of their contacts at our firm in an attempt to figure out what their former analyst will be doing. In addition to being highly disruptive, this usually leads to our employees (often accidentally) leaking confidential information that they should not. As a general rule of thumb, the less your competition knows the better, and your former analyst's incentives are now aligned with the new opportunity, not the old one. 

Usually this kind of PM stops caring a few months after resignation anyways, and moves on, so even a little bit of cool off time is plenty to prevent this from happening.