Aaron Bertrand explains a bug in SQL Server 2012 and 2014 which can crop up after dropping a column:
A fix has been published for SQL Server 2012 (SP2 and SP3), and one is coming for SQL Server 2014 (RTM and SP1) – though in this most recent update, SQL Server 2014 was removed from the list of affected versions (not sure it was accurate to remove those). I will update this space once the next 2014 CUs are released, but for now you should plan to rebuild tables after dropping columns as a matter of course and, more importantly, as part of the same maintenance operation.
This is a good reason to stay up to date on CUs.