Thread:KrytenKoro/@comment-31365078-20200221061655/@comment-2095950-20200221221638

Digimon Collectors also had some cards featuring Summon Night characters.

"weird they went so long without appearing in stuff of other franchises only to suddenly do a bunch at once"

I think it's because promotional crossovers are popular right now. Symphogear XD Unlimited has already had three crossover events already. Revue Starlight -Re Live- had a crossover event with Symphogear, Schoolgirl Strikers had a crossover event with Fate/stay night, some game I never really learned the name of had a crossover with Action Heroine Cheer Fruits (that I don't know if it was an event or what, I just remember the anime's Twitter mentioningthe the crossover), and Granblue Fantasy has had crossovers with but Love Live and Precure'' and probably more franchises because if I'm not mistaken it's the most popular gacha at the moment.