The RAE says:

[La preposición "a" con complementos directos se usa] ante nombres de cosa, cuando son complemento directo de verbos que significan orden lineal o jerárquico, como preceder o seguir, y otros como acompañar, complementar, modificar (en el sentido gramatical de ‘servir de adjunto o complemento’) o sustituir (en el sentido de ‘ocupar el puesto [de otra cosa]’): El otoño precede al invierno; La calma sigue a la tempestad; El adjetivo modifica al sustantivo; El aceite sustituye a la mantequilla en esta receta.

The reason for this has to do with the flexibility of Spanish:

• El trueno siguió la tormenta = The thunder followed the storm
• El trueno siguió a tormenta = The storm followed the thunder

In Spanish you can easily put the subject at the end of a sentence, so "a" is added to the direct object to prevent misunderstandings.

Is it easier to think of "seguir" as "to follow" and "seguir a" as "to be followed by"?

"El trueno siguió la tormenta" = "The thunder followed the storm". "El trueno siguió a la tormenta" = "The thunder was follwed by the storm" ?

Just wondering about what it really means to be transitive or intransitive here. (I probably ought to look up "seguir" first but...)

Isn't - even in English - the "follow" of "to follow someone down the street" different from the "follow" that the rain does after the thunder? I mean, the rain doesn't "track" the thunder. Maybe that is why it is "intransitive" in the "a" case and not in the other one.

Now I will turn to my dictionary....

Reading Lazarus response, I wondered why my dictionary should show that "A". (I capitalize it here on purpose because, indeed, what is shown is a very small capital A).

In other words, I was finding it difficult to square the dictionary entry:

seguir vt (en el tiempo) to follow; ~ A (remember, this A is very little) algo/algn to follow sth/sb; los disturbios que siguieron a la manifestación

with the possibility offered in Lazarus' response to say that the thunder followed the storm: "El trueno siguió la tormenta." -- no "a"!

However, I had also noticed that the preposition "de" is sometimes written "DE" (again, small) to go with a verb and other times not. Nor is the "a" always written as a little capital. Sometimes I see "a".

So perhaps the dictionary is indicating to me with the little capital "A", that whether or not I am to use it depends. Unfortunately it does not tell me what it depends on!

Perhaps Lazarus answer has done that now. I guess I cannot ask that the dictionary also be a grammar reference. (...Uh oh...I still haven't ordered a grammar text....I had better stop posting until I do that!)

Whew...