I think sometimes they just try to find something wrong to fire people. I worked at KMart once as a teenager for the holidays. At about the second or third week a girl who had worked there previously for the holidays came in and was a bit sad that she couldn't be hired, we had too many workers already.
. Well about a week later, they let me go stating that I spoke too softly to customers, (what !) me the kid who the teachers who often moved due to talking.

. And, also I had had too many 'voids' (customers who changed their mind kind of thing). Well for the past week I had only had two, maybe three a day, and all were because customers changed their minds, not because I made a mistake. And a lot less than the regular workers. Truly strange I thought.
Well about two weeks after that I was shopping there, when I realized that the girl who had not been hired earlier, was now working there. I think she just wanted the job that badly that she got management to let me go and hire her.
Though I missed the money at the time, I've always been thankful I never stayed for that dead end job.

And, of course what do I do now, I work the window with the PO. I don't have any problems with being too quiet or making lots of mistakes.

Kim
BTW, good luck in finding a new job!