[web2py] Unwanted submit on checkbox click ?

[BrendanC]

I'm building a dynamic table with a checkbox in each row with the code 
below. The table displays correctly and html looks ok.
However when I click on any checkbox, the page is being submitted - not 
sure why.
Any suggestions?

    for item in imgtags:
      s = [[item.name , '<input id="%s" />' %item.name]]
      chbx = (INPUT( _type='checkbox', _name= 'chk%s' %item.name, 
_onclick="alert(this.checked)"))
      arr.append ([[item.name],[ chbx]])
  
    return XML(TABLE(*[TR(*r) for r in arr], _id = 'tagtable'))

BrendanC