I would think that the pressure on Singtel to keep paying off dividends has also caused its leverage and credit risk to worsen also. It's dividend payout ratio is actually 77% for 2018 Q3, quite high! If Singtel were to cut back on dividends, its share prices will really tank since the dividends have all been priced into the shares.
Maybe just to wrap Sandra point up, the measure of leverage that was talked about in the article was Debt/EBITDA. If more debt is taken on, but EBITDA decreases, the leverage levels will increase. That's probably why they downgraded Singtel's rating.
Moody downgraded Singtel's rating due to concerns over price competition (lower average revenue per user and profitability) in Singapore and Australia and its weak expections for Singtel's underlying EBITDA (earnings before interest, tax, depreciation and amortisation) for the next one and a half years. The second reason for the downgrade is the potential increase in risk from subscription to India's Bharti Airtel right issues. The subscription would weaken Singtel's metrics and keep net leverage above Moody's tolerance for the rating.