This is a good question. At first I thought it was because it was 60 percent of the body, but no quorum rule I know of is written like that. So ... Wisconsin Senate Rule 15. http://nxt.legis.wisconsin.gov/nxt/gateway.dll?f=templates&fn=default.htm&d=rules&jd=senate%20rules
Senate Rule 15. Roll call, quorum. Before proceeding to business, the roll of the members shall be called, and the names of those present and those absent shall be entered on the journal. A member present during any part of a roll call day shall be included in the official attendance roll call for that day. A majority of the membership presently serving must be present to constitute a quorum for the transaction of business; a smaller number, however, can adjourn and may compel the attendance of absent members. When a roll call discloses the lack of a quorum, further business may not be conducted until a quorum is obtained, but the members present may take measures to procure a quorum or may adjourn. So, it's not a quorum issue. There are 19 Republicans and 14 Democrats, so unless there are only 16 Republican votes, a quorum exists.
A quorum is the minimum number of members of a deliberative assembly necessary to conduct the business of that group.
A quorum may be a majority, some number greater than a majority, or some number less than a majority.
The number of members that constitutes a quorum differs depending on the assembly and is usually provided for in that assembly’s governing documents (for example, its constitution, charter, or bylaws). The quorum may also be set by law.
Apparently, in the WI Senate, 20 members are required to have a quorum and not just a majority of 17.