Why would a "ten bagger" refer to baseball? Where is the ten involved in it?
The Wikipedia link given in Adel's answer is spot-on, but the additional explanation misses the mark.
Sports (and martial) metaphors are big in the business world. In baseball slang, a "two bagger" is a double and a "three bagger" is a triple. So an investment that doubles your money would be a "two bagger", and one that triples your money would be a "three bagger". By extension, an investment that returns ten times your investment would be a "ten bagger"—even though that's quite impossible in baseball (at least under the current rules).
Well, it was a new word based-on the words "two-bagger", "four-bagger" etc.
Why 10? Hmm, because a ten-fold increase is impressive, and easy to say - monosyllabic. And maybe $10.00 bill is nice.