They're old and the schedule is condensed, so their energy level is understandably inconsistent.
The lineups have been very inconsistent due to injuries.
The team generally relies on jumpshots, meaning they're vulnerable to hot streaks and cold streaks . . . especially in the 4th quarter, when the old legs get tired.
Being a poor rebounding team means it's easier for opponents to make comebacks, and there are fewer second chance points to make up for mistakes and misses.
Certainly doesn't help that the production from the center spot is generally very weak. JO is almost entirely a one-way player now.