Could be a sign that they're doing so well it's not possible to invest it all effectively. Could also be a sign that they are transitioning to a mature company

Could be a sign that they're doing so well it's not possible to invest it all effectively. Could also be a sign that they are transitioning to a mature company, where hoarding money means having a long runway to respond to market changes without going under (or begging investors for capital to keep you afloat).