Why does INSERT hang on S mode (shared) locks?

I thought INSERTs used "X"  mode (exclusive) locks. Does it also acquire "S mode (shared) locks?

Here's why I ask. I'm capturing extended events for lock_acquisition > 1 second and see lots of waits in the range of about 5 seconds (5000000 µS), always for the same INSERT on the same table, and the mode is always "S". 

Chuck