Cross *really* wanted to stay with the Rams - he loved playing for Allen - but Jerry Williams was an old friend. When Williams, who'd just been hired by Philly, gave him the opportunity to be a player / coach with the Eagles in 1969. It was something Cross really wanted to do and agreed to the deal. He went back to Philly in exchange for Boomer Brown.
Cross told me that, during Rams' practices in '68, neither he nor Clancy Williams could stay anywhere close to the same zip code as Jackson, who spent most of that year on the Rams' taxi squad. So, when Cross got back to Philly, he started pushing Pete Retzlaff, who was his former teammate and the Eagles' GM at the time, to trade for Jackson. Because Jackson was so young ... Allen didn't think twice about moving him.
Retzlaff was thrilled to get rid of Lang (and Brown, truth be told) and Jackson turned out to be an absolute steal.