We have 2 x pony tail trees in front garden. been there for 4 years doing well . Not in pots ,placed in ground. One is losing its
green leaves . They are falling off? Sandy soil well drained area.
Certified GKH Gardening Expert
Has it been any more wet this year than normal? This could cause the issue, as well as wind. This will be the usual culprit. Otherwise, after these options will come to fertilization, which should be done a handful of times per year.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/houseplants/ponytail-palm/ponytail-palm-care.htm