Could also be the unit wasn't tightened down correctly. There was a lot of that going around not long ago, units coming from the factory with the AC hold down bolts loose enough to wiggle with your finger.
It could also be a combination of things. It could be the drain is clogged, or it's too close to the roof and is blocked. Both of those are issues that have been addressed in this forum by various members.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!