She might not have been ready for it to end. Sometimes, women can experience refactory periods after climax just like men---where continuing on can be painful or uncomfortable after orgasm. If you weren't ready to climax as well, switching her position may have been the female equilvalent of reciting baseball stats to prevent coming too soon, that way she could last long enough for you.
On the other hand, she may not like the loss of control. Climax (when you're experiencing it) is a force of nature, but to have one, you have to let go. So, if your wife is very concerned with control over her emotions and body, she may be preventing climax because she doesn't want to lose it (which is what a good orgasm tends to do).
Another possiblilty is that at some point in your past sexual history, she DID lose control...of everything. Not just muscle spasms, but her bladder as well. At risk of TMI, that happened once with me. My fiance literally rocked my world, and I was having multiples like crazy...and then I went one orgasm too far and lost it completely. I was nearly ready to die from emabarrassment. It took a while before I was comfortable enough to orgasm again---I was terrifed I'd wet myself. Now, if I think I'm getting close, I'll tap out...but immeadiately after it happened, experiencing climax was something I wanted to avoid at all costs because I didn't want to risk another accident.
The last idea I have is that it might be a trust/self image issue. You really have to be comfortable and trust a person to let go and let them see you at your most vulenerable. If your wife isn't comfortable with her looks, or isn't comfortable with your relationship, it might be another reason for her behavior.