Without knowing the specifics it can be any number of things. It could be anything from waiting on an inspection to be done, a delay in materials being delivered, contractor scheduling issues, design change, etc.
Two main reasons:
1. Construction is heavily divided up into sub-contracts. For example; masonry is a different company then metal flashing, then roofing, then painting... So, a subcontractor comes in to install the brick, then it may be a month before the next subcontractor is scheduled to come in.
2. Construction is scheduled not only to sequence the work for best installation (metal deck has to go on before roofing), but also to best protect the work that is installed. They won't put stainable final cladding on the buildings until the paving work is done so as not to get it discolored by the earth. They won't do the paving work until all heavy equipment has rolled through (so as not to crack it).
Both great answers, and correct.
When a construction schedule is produced, a number of the tasks land on the "Critical Path". These tasks must be completed when scheduled in order to keep the project - as a whole - on track. Tasks not on the critical path have a little bit of leeway and can be put off until later without affecting the schedule.
For example, you can do exterior painting just about any time, since it's the last level of finish applied and rarely affects other work.
Knowing the contractors that are working on FLE, and knowing Disney's in-house construction management department, you can rest assured that everything is being done exactly when it needs to be done, and in the most logical order.
When you see "random" parts of exterior work being done, it could simply be that those crews finished up a portion of interior work, went outside to work while the weather was nice, and then went back inside once someone else's work was completed and out of their way.
A lot of construction, especially tedious exterior finishes like Disney uses, can only be done when the weather is perfect (dry, between certain temperatures, etc).
So many variables. You'll never find a construction project where all of one task was completed in its entirety before moving onto the next.