This is only based on my experience and that of about 5 others here on myfico, but I normally PIF on my Alaska Airlines card each month. BoA soft pulls EX about every 2 or 3 months.
I've never seen BoA soft pull EQ before, but they did so very shortly after I did a BT of $1,500 to that card.
Lenders can and do SP any bureau, I understand. But I've been a BoA customer for years and this is the first time I've seen them SP EQ. Their pull patterns at least with me have been very predictable - once every 2-3 months on EX.
I've spoken with abut 5 other myfico members and they noticed the same pattern shortly after doing a BT. Several mentioned that BoA started SP'ing EQ monthly (in addition to EX) after their BT. Just interesting to note.
Certainly many factors are at play in everyone's profile, and it's difficult to look at one in isolation of any others.
It may not happen to everyone, and YMMV, but sharing the information in any event. 