I use paper/corrugated substrate for mine with hides but when I did use aspen I saw the behavior from time to time. Burrowing is a very normal behavior for boas.
If a boa doesn't have a hide, it may burrow to feel secure.
If it has a hide and deep enough substrate it may do either or both (use hides, burrow or burrow in the hide).
They also will simply hang out in the open happily enough, especially once larger.