CV Axle Popping out of Transmission?

Hi,
I have a 1998 Honda Prelude Base Model Turbo H22A4 5spd MT with 51,000 miles. I have replaced both OEM Honda Axles with different brand axles. The driver side axle seem to go in pretty good. And the passenger side gave me a little problem when it went in. This was the first axle. This one popped out when I was driving and car wouldn't go anywhere, pulled the axle out and the splines on the end of the CV Axle were stripped from the tranny grinding on them. Then I went back to the place to get the new axle replacement, and when I went to put in the new axle, it went in really easy. Easier than the first one. So this being my second new axle, I drove it around for a total of 60 miles and I did get on the gas a few times but when driving in regular traffic, I pressed the gas to go forward and the axle popped out again. Towed it home and took everything apart again and the axle on the passenger side was stripped on the cv axle splines again on the end. Some reason the axles seem to keep popping out of the transmission. My question is, Why does the new axles keep popping out the transmission? Hope all that info can help you out.

Thanks,
Jay