The only time I have seen the cursor jump "wherever" was when inserting a link. After the link is inserted I have grown accustomed to "WTH is the cursor now" syndrome. Most times the cursor will jump near the beginning of the post, and it seems to always insert an unwanted carriage return after the link. The only way I have found to mitigate this is to insert carriage returns on purpose and complete the sentence BEFORE inserting a link anywhere.
 
Not sure if related to what you are seeing, but that happens to me all the time (and I am on desktop).

This used to happen to me all the time on my work laptop. I have a Lenovo Yoga 2. Believe me, it's the touchpad. I'd be typing something and then all of a sudden the cursor would jump to some earlier part of the text and start inserting letters into the existing text. I hit the freaking roof every time it happened. Sometimes you can touch the pad accidentally with your palm without even realizing, despite how careful you're being. 
 
Look in the settings for the trackpad. If it's a Synaptics device like mine, there should be a setting somewhere called "palm tracking" or "PalmCheck." If you set that to maximum, it will recognize when you've accidentally brushed the pad with your palm and reject it. Even if you're confident you're not hitting the touchpad, try this setting anyway. 

Never had to deal with a laptop so I'll assume at least one of the answers above helps, but I've noticed this behavior on my desktop PC and there's usually two ways it can occur (one may apply to a laptop).  The first is when there's dust and gunk that builds up on the mouse (I've got a thumb trackball so this happens about once a week).  The second is when the input (mouse, trackball, pad, etc.) ends up on a USB port that is shared and some other device is getting too much priority.  Always good to keep the mouse on its own port.

Jewellery is a common cause of this. 
 
My Dad was ready to throw his laptop through the window until I pointed out that every time he reached over the trackpad to type, his gold chainlink bracelet was moving the cursor.  Metal watch straps can do the same.

(Note to self: Check old photos and see if I can find any photographic evidence that anyone has ever seen Karyn's dad and P Diddy in the same place at the same time.)

Thanks to all for the suggestions.  I'll try all of them!!

After some googling and laptop sleuthing I found a way to disable the touchpad (I use a mouse).  Now let's see if that solves the problem.  Fingers crossed!!

I hate laptop touchpads. I'm constantly making mistakes with them. And hitting the right button is super awkward. 
 
When I got my Lenovo I didn't think for one minute that I'd be using the touchscreen, but I find myself using it more and more.

It's weird, but every time I stand up I lose my lap top...

You could also have a stuck key on your keyboard. The tab key for instance.