When following delegation iteratively as specified with the +trace option, dig will begin by requesting the NS records for the servers authoritative for root ("."). These may or may not be supplied with glue - that is A and AAAA records that can be used for the next queries dig has to send. When there is no glue provided, either on the initial query for the root nameservers, or later on when following delegation, dig will revert to recursively querying the servers listed in /etc/resolv.conf to obtain the needed A/AAAA RRsets.
Specifying @server does not change this behaviour - the server specified in this way will only be queried for the NS records for the servers authoritative for root (".").