Sometimes subscribe buttons load a new page with JSON code rather than new page

I've seen this strange behavior in several modules, including Views and Simplenews. Here is what the JSON code looks like in Simplenews:

{"status":true,"data":"\n \x3cdiv class=\"defaultblock\"\x3e\n \x3cdiv class=\"blockcontent\"\x3e \x3cp\x3ePlease join our mailing list. We will only send you important updates about Rainbow Rumpus. Thank you!\x3c\/p\x3e\n \n \x3cform action=\"\/quicktabs\/ajax\/block\/simplenews_delta_1\/1\" accept-charset=\"UTF-8\" method=\"post\" id=\"simplenews-block-form-1\"\x3e\n\x3cdiv\x3e\x3cdiv class=\"form-item\" id=\"edit-mail-wrapper\"\x3e\n \x3clabel for=\"edit-mail\"\x3eEmail: \x3cspan class=\"form-required\" title=\"This field is required.\"\x3e*\x3c\/span\x3e\x3c\/label\x3e\n \x3cinput type=\"text\" maxlength=\"128\" name=\"mail\" id=\"edit-mail\" size=\"20\" value=\"dfdfg2DGT.COM\" class=\"form-text required error\" \/\x3e\n\x3c\/div\x3e\n\x3cdiv class=\"form-radios\"\x3e\x3cdiv class=\"form-item\" id=\"edit-action-subscribe-wrapper\"\x3e\n \x3clabel class=\"option\" for=\"edit-action-subscribe\"\x3e\x3cinput type=\"radio\" id=\"edit-action-subscribe\" name=\"action\" value=\"subscribe\" checked=\"checked\" class=\"form-radio\" \/\x3e Subscribe\x3c\/label\x3e\n\x3c\/div\x3e\n\x3cdiv class=\"form-item\" id=\"edit-action-unsubscribe-wrapper\"\x3e\n \x3clabel class=\"option\" for=\"edit-action-unsubscribe\"\x3e\x3cinput type=\"radio\" id=\"edit-action-unsubscribe\" name=\"action\" value=\"unsubscribe\" class=\"form-radio\" \/\x3e Unsubscribe\x3c\/label\x3e\n\x3c\/div\x3e\n\x3c\/div\x3e\x3cinput type=\"submit\" name=\"op\" id=\"edit-submit\" value=\"Save\" class=\"form-submit\" \/\x3e\n\x3cinput type=\"hidden\" name=\"form_build_id\" id=\"form-3f826225e2214b9c2bb20e8db2be8c2a\" value=\"form-3f826225e2214b9c2bb20e8db2be8c2a\" \/\x3e\n\x3cinput type=\"hidden\" name=\"form_id\" id=\"edit-simplenews-block-form-1\" value=\"simplenews_block_form_1\" \/\x3e\n\n\x3c\/div\x3e\x3c\/form\x3e\n \n \n \n \x3ca href=\"\/taxonomy\/term\/1\/0\/feed\" class=\"feed-icon\"\x3e\x3cimg src=\"\/misc\/feed.png\" alt=\"Syndicate content\" title=\"Rainbow Rumpus newsletter feed\" width=\"16\" height=\"16\" \/\x3e\x3c\/a\x3e \x3c\/div\x3e\n\x3c\/div\x3e"}

Why is this happening? What do I need to check to make sure it doesn't happen again?