Contrary to what someone suggested, I don’t think all MO use the Israeli/Sephardic pronunciation; some do, and some don’t.
The question of why some switched to the Israeli pronunciation is an interesting one. Some of them probably spent time in Israel and came to get used to it. I have relatives who spent time there as a child and for them, it just sounds right and the Ashkenazi pronunciation sounds annoyingly American (particularly the american R’s and putting the emphasis on the second to last rather than the last syllable). Many of those who were baalei teshuvah learned Hebrew as adults, and, let’s face it, it’s a lot easier to learn the Israeli pronunciation, since there are fewer sounds to learn.
Regardless, many people feel very strongly, for whatever reason, that their way is correct. So if it’s easy for you and makes your parents happy, why not pronounced sav’s as tav’s around them? I personally switch back and forth depending on who I’m talking to.