Why was the traditional character 滾 simplified to 滚? It's only one stroke less so it seems doubtful that it was done to make the character simpler to write. Does it change the meaning of the character in some way or is there any other logical reason why this character was simplified in this fashion?
『滾』 is the formal form of gun3. 『滚』 is just used in normal people (like 『閱』『門 + 兊』(yue4)). This difference could be seen anywhere before. These days there are still many differences between Traditional Chinese and Simplified Chinese like 『強』『强』(qiang2). (I am not sure 口 is formal or 厶 is formal.)
Simplified Chinese just chooses the forms written by normal people（『俗體字』） first. So then, 『滾』 is simplified to 『滚』.
Not a real answer (i.e., a total guess), but too long for a comment:
Zhongwen.com (a useful resource for issues etymological) says that the traditional character 袞 ("imperial robes") comes from the characters for official (公) and clothes (衣), with the bottom part of 公 altered to 口. As many simplified characters are based on common alternate/handwritten forms, it is possible that the variant with an un-altered 公 survived because it appears to match the meaning of 衮 better than the form with the 口. The simplification committee may have followed that precedent when deciding on the simplified form.
(The simplified form of 滚, of course, would naturally follow.)