There are several common reasons for particles leaving the defined domain. Here’s a few previous forum threads covering this topic:
Please see https://mfix.netl.doe.gov/forum/t/sigsegv-segmentation-fault/2631/2
and
https://mfix.netl.doe.gov/forum/t/error-invalid-memory-reference/2322
If none of the above helps, please follow up here. Thanks.