I suspect that it is only happening on buttons added to the database before the file was converted to Filemaker 12 format. The programmers, in my opinion, got a bit lazy when they wrote the conversion code converting the .fp7 layout into .fmp12 format.
For More Information see: Buttons spontaneously change size and position when a text style is applied
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip