[t a] will convert bang, float and symbol messages to list mesages.
this becomes problematic, if the objects that receive the data from [trigger] have methods for e.g. both "list" and "bang" and use different semantics for those two messages (e.g. "bang" triggers the object's output, whereas "list" sets its internal state - an empty list could be used for clearing the state)
it also makes triggering of the empty-lists-discarded bug (fixed in #3606330) very easy, e.g the following will not do anything:
[pack 0 0]
the attached patch sends the incoming messages unmodified to any "a" outlet of [trigger]