Trees are barely a firm category of plant at all. It's basically just tall plants with woody stems. Plants can gain and lose woody stems without too much trouble (relatively speaking, over evolutionary time). So any time a plant species currently growing soft stems can benefit from being really tall, they have a good chance of evolving into "trees".

Not OP, but:

https://eukaryotewritesblog.com/2021/05/02/theres-no-such-th...

One example is oak trees being more closely related to tulips than to pine trees.

(Tulips and oak trees are both angiosperms, flowering plants, and share a common angiosperm ancestor. Pine trees on the other hand are gymnosperms.)