Was it my rooting that caused my Android to malfunction? Or what was it really? I finally rooted my phone around a week ago, amidst all those battling feelings, and I used it for around 4 days.

While the phone was charging through the USB port of my computer, I remember like putting the computer on suspend and then removing the phone. Thereafter, it never turned on! Except sometimes, where it came to the boot screen and got stuck, or it went just past the boot screen and got stuck again, or where it didn't even turn on the screen but just kept vibrating.

It was only charging through the USB, so I had not enabled file sharing or anything where I would have to disconnect the phone from the computer first before unplugging, so I'm quite sure that it's not anything to do with corruption of data due to just unplugging.

My Android (ICS) is yet under warranty, although rooted, so it's currently at the company service centre getting repaired. Their warranty card has got a whole bunch of terms and conditions that void warranty, but nothing to do with rooting thankfully. I only rooted it, and went NO further into flashing another ROM.

So my question, what happened to my Android? Was it some application that required root, misbehaving? Or was it due to overcharging? It can't be due to overcharging since when I plugged in my phone it was only about 60-80% charged, and I was at the machine for only a few minutes before disconnecting it from my computer.

Whatever the reason is, please help me figure it out. The phone comes back this coming up Wednesday, and I want to be knowledgeable enough to understand what's going on and whether I should root it again if the company has unrooted it (hopefully not! ;) ).
It's pretty unlikely that rooting will ever cause hardware to malfunction--software maybe, but even then unlikely. 

In the event that you think a rooting process has caused your device software to malfunction it is advisable to attempt a full factory reset--since you had a hardware issue the device would still be malfunctioning. 

Edit: As Dan correctly notes below this is not the same as a 'Factory data reset' that you can find in the phone menu, it is more akin to flashing a stock rom including the recovery and often bootloader(and sometimes specialized software will be required). It looks like the device mentioned might be OEM by ZTE, maybe this thread would have a stock rom http://forum.xda-developers.com/showthread.php?t=1353526 )

One thing: It might be good to know what is actually happening with the root(maybe try to find a 'manual' root process) as some root procedures are more invasive/risky than others.