agg function in columnstoreindex

i use sql server 2014 sp1. i build a clustered columnstore index on a table I found something interesting, when i write a query and have aggregated function on top of it, like avg etc., then the optimizer doesn't use batch execution mode, but when i take the aggregation function off and store it into a temp table and do agg in a second step, then he uses batch mode and query much faster. Can anyone explain this behavior?