You told, that becoming hot microcontroller retaint program. How did you test this? I ask, because if device correctly runs in normal application an ESD damage is rather unprobable. Then, some timing mistake of your programmer is more probable.
To ESD problem: Sensible electronics can be destroyed when even having no contact to device. Even if there is no discharge of static electricity! Just by coming near the device. How? By Influencing. Big static electricity is origin of very strong static electrical field. Bringing this rapidly near a device results in a heavy pushing (or pulling) of electrons, possibly resulting in a destroyed device.
That's the reason why plastic boxes are so dangerous for archiving and stocking chips. Only the opening of box is enough to destroy the chip.
When touching a PCB, first search for massplane or parts being connected to massplane. Allways, first touch PCB at such point, before handling PCB. Be aware that there is no chip very close to this point.
The best, of course, would be the use of those many anti-electrostatic tools for achieving compensation of potential difference. But if this is not possible use 'Touching-grounded-parts-first-methode'.

Bye,
Kai