What happened ? PIC attached

I'm gonna guess contamination in the primer or metal under the paint.
Could be oil, water or incompatible materials; often takes a while to show up.
Doesn't quite look like body solder but that's possible, depending on where it is and the history of bodywork and/or paint.

I hired an experienced body /painter to do this. It's popping up in different areas - painted 2 years ago - whats the best approach to fix or does it need to be all redone.

The correct fix is to sand into it and find the problem, you'll need to go at least into the primer if not down to bare metal.
Look for signs of rust (water in air lines, or getting on car during repairs).
Also, an excessive amount of primer or polyester could be the issue.
Check the whole car over very thoroughly, and find out what refinish system (or different products) were used.
On occasion, a shop painter that's certified in a given system, and can prove that to the tech rep, will stand behind the work.
Other times, unfortunately, not so much.

I am sorry to say but in my opinion with the knowledge that this is two years old. It is the base metal rusting under the paint and primer, could be rusting through from the back side or the surface metal wasn't prepped/ treated/ painted properly when it was done

PPG certified painter here.
from looking at the pic. to me it looks like the clear coat is de-laminating from the base coat. there might be some contamination, like dirt, oil, moisture, on top of the base

is it only happening in 1 spot or all over the car?
how long was the base coat open before clearing?
what type of air supply equipment when painted?

i myself would first start with talking to the person painting the car and see what he thinks.

Its happening in a couple spots. Its unlikely rust because its on 2 different panels (so far) and both were pristine. I picked up the car about 2 years ago and garaged it - that's it - I did call the painter - and sent pics - haven't heard back. I looks like specs on the clear coat - almost like sap after you park under a tree - a few places - you can feel it better then you can see it

it is not de-laminating clear, Clear-coats look milky when they separate from the base-coat, the lifting is happening under the paint and/or primer. I would say it is spider rust forming

a physical inspection is required to really diagnose it best.

in my opinion it looks like moisture contamination to me.
15 years ago a shop a old friend worked at had a similar problem. it was actually one of the prep men use shower to shower . the talcum from his hands transferred to the body and it worked thru to the top coat causing a similar problem. ever since i always use a solvent based AND a water based pre-cleaner before painting.

to repair will require a strip and re-paint.

The pictures appear to be blisters. Tree sap will normally eat into the paint leaving a slight depression behind.

As the others have said, it's likely contamination of some sort under the paint but could also be due to incompatibility of of materials used.
What layer it is at can only be established by carefully peeling back the blister. The layer where the peeling stops and the paint stays behind (IE: Primer , spot putty etc is where the problem is.
The contamination is creating a gas (pressure), that in turn lifts the paint stretching it. While doing so it creates the bubble.\
Parking the car in the hot sun usually accelerates the process.
Depending on how many areas are affected and the root cause will determine the repair process.
A complete strip to bare metal and repaint may be the only sure way to eliminate future issues.

What you have pictured is one way it can look when it starts to rust under the paint, usually where there's filler of some sort. Not always but usually if it's a chemical reaction or contamination there are signs of it starting before 2 years.

Without a dought