I've had this happen on my ETS150 too, and once on my TS55. It seems to stem from a bad electrical connection (causing arcing between the plug & the socket pins) but once it happens it gets steadily worse, and can then 'infect' other tools due to the corrosion. The arcing causes burning and carbon build-up on the contacts, which then leads to higher resistance, which causes heat build-up, which causes more burning, etc...
The first time I noticed it was on my ETS - it got to the stage where the sander would even completely cut out every few seconds. I changed the plug-it socket in the handle, but then used the same plug-it lead on the new socket and on my TS55, and the problem re-occurred on both.
Eventually I replaced the sockets on both tools, and replaced the lead, at the same time.
I think (in my case) it stems from using the same plug-it lead, constantly swapping it between different tools, and it eventually wears out (I know, that's the point of having it!). The last time it happened I caught it before it 'infected' anything else. This time, instead of replacing the whole plug-it cord (which is bundled together with the hose, so a pain to swap out) I just cut off the plug-it and fitted a replacement plug on the cable. The replacement ones seem a lot sturdier, with heavier-duty contacts inside. I've not had any further problems with it.
It's always inconvenient when it happens (like any failure) but it doesn't happen so often that I feel the need to hardwire anything - for me the convenience of the plug-it easily outweighs the downside.