I see you figured out the problem already so just a couple comments FYI.
APRS-IS shows your last packets on 3/15 having a packet path of "WIDE1-1, WIDE3-3". In the Sea-Tac area there are plenty of digis and a simple WIDE2-1 will get your packet gated and avoid congestion on RF. Your packet rate also looked a biit high. Digis are sometimes known for ignoring stations they consider abusive. However digi software has improved - with those digis if they see your packet already duplicated on RF or APRS-IS they can avoid dupping it again. For example BALDI which is south of you with wide coverage does this.
Also if you haven't already set transmit power to low - no need for 50 watts if digis are nearby and the goal is to let someone know where you are on aprs.fi.
Remote places like Death Valley I use "WIDE1-1, WIDE2-1" and increase tx power to high. The digis there are really wide and one hop from them will get it to an i-gate, or your buddies over in the next valley. That doesn't mean WIDE3-3 isn't useful just take into account location and how many hops it might take to get to an I-gate.
Have fun.